Timeless protection of telomeres

Navigating sex and sex roles: deciphering sex-biased gene expression in a species with sex-role reversal (Syngnathus typhle)

Sexual dimorphism, the divergence in morphological traits between males and females of the same species, is often accompanied by sex-biased gene expression. However, the majority of research has focused on species with conventional sex roles, where females have the highest energy burden with both egg production and parental care, neglecting the diversity of reproductive roles found in nature. We investigated sex-biased gene expression in Syngnathus typhle, a sex-role reversed species with male pregnancy, allowing us to separate two female traits: egg production and parental care. Using RNA sequencing, we examined gene expression across organs (brain, head kidney and gonads) at various life stages, encompassing differences in age, sex and reproductive status. While some gene groups were more strongly associated with sex roles, such as stress resistance and immune defence, others were driven by biological sex, such as energy and lipid storage regulation in an organ- and age-specific manner. By investigating how genes regulate and are regulated by changing reproductive roles and resource allocation in a model system with an unconventional life-history strategy, we aim to better understand the importance of sex and sex role in regulating gene expression patterns, broadening the scope of this discussion to encompass a wide range of organisms.

The TIMELESS Roles in Genome Stability and Beyond

TIMELESS protein (TIM) protects replication forks from stalling at difficult-to-replicate regions and plays an important role in DNA damage response, including checkpoint signaling, protection of stalled replication forks and DNA repair. Loss of TIM causes severe replication stress, while its overexpression is common in various types of cancer, providing protection from DNA damage and resistance to chemotherapy. Although TIM has mostly been studied for its part in replication stress response, its additional roles in supporting genome stability and a wide variety of other cellular pathways are gradually coming to light. This review discusses the diverse functions of TIM and its orthologs in healthy and cancer cells, open questions, and potential future directions.

Heritable defects in telomere and mitotic function selectively predispose to sarcomas

Cancer genetics has to date focused on epithelial malignancies, identifying multiple histotype-specific pathways underlying cancer susceptibility. Sarcomas are rare malignancies predominantly derived from embryonic mesoderm. To identify pathways specific to mesenchymal cancers, we performed whole-genome germline sequencing on 1644 sporadic cases and 3205 matched healthy elderly controls. Using an extreme phenotype design, a combined rare-variant burden and ontologic analysis identified two sarcoma-specific pathways involved in mitotic and telomere functions. Variants in centrosome genes are linked to malignant peripheral nerve sheath and gastrointestinal stromal tumors, whereas heritable defects in the shelterin complex link susceptibility to sarcoma, melanoma, and thyroid cancers. These studies indicate a specific role for heritable defects in mitotic and telomere biology in risk of sarcomas.

Telomere Replication: Solving Multiple End Replication Problems

Eukaryotic genomes are highly complex and divided into linear chromosomes that require end protection from unwarranted fusions, recombination, and degradation in order to maintain genomic stability. This is accomplished through the conserved specialized nucleoprotein structure of telomeres. Due to the repetitive nature of telomeric DNA, and the unusual terminal structure, namely a protruding single stranded 3' DNA end, completing telomeric DNA replication in a timely and efficient manner is a challenge. For example, the end replication problem causes a progressive shortening of telomeric DNA at each round of DNA replication, thus telomeres eventually lose their protective capacity. This phenomenon is counteracted by the recruitment and the activation at telomeres of the specialized reverse transcriptase telomerase. Despite the importance of telomerase in providing a mechanism for complete replication of telomeric ends, the majority of telomere replication is in fact carried out by the conventional DNA replication machinery. There is significant evidence demonstrating that progression of replication forks is hampered at chromosomal ends due to telomeric sequences prone to form secondary structures, tightly DNA-bound proteins, and the heterochromatic nature of telomeres. The telomeric loop (t-loop) formed by invasion of the 3'-end into telomeric duplex sequences may also impede the passage of replication fork. Replication fork stalling can lead to fork collapse and DNA breaks, a major cause of genomic instability triggered notably by unwanted repair events. Moreover, at chromosomal ends, unreplicated DNA distal to a stalled fork cannot be rescued by a fork coming from the opposite direction. This highlights the importance of the multiple mechanisms involved in overcoming fork progression obstacles at telomeres. Consequently, numerous factors participate in efficient telomeric DNA duplication by preventing replication fork stalling or promoting the restart of a stalled replication fork at telomeres. In this review, we will discuss difficulties associated with the passage of the replication fork through telomeres in both fission and budding yeasts as well as mammals, highlighting conserved mechanisms implicated in maintaining telomere integrity during replication, thus preserving a stable genome.

A Timeless Tale: G4 structure recognition by the fork protection complex triggers unwinding by DDX11 helicase

How the replisome senses and deals with DNA secondary structures has been a mystery. A new study from the Sale and Pellegrini laboratories finds that the Timeless protein has a G-quadruplex binding domain that works together with the DDX11 helicase to facilitate replication of G4 DNA structures.

Insights About Circadian Clock and Molecular Pathogenesis in Gliomas

The circadian clock is an endogenous time-keeping system that has been discovered across kingdoms of life. It controls and coordinates metabolism, physiology, and behavior to adapt to variations within the day and the seasonal environmental cycles driven by earth rotation. In mammals, although circadian rhythm is controlled by a set of core clock genes that are present in both in suprachiasmatic nucleus (SCN) of the hypothalamus and peripheral tissues, the generation and control of the circadian rhythm at the cellular, tissue, and organism levels occurs in a hierarchal fashion. The SCN is central pacemaker comprising the principal circadian clock that synchronizes peripheral circadian clocks to their appropriate phase. Different epidemiological studies have shown that disruption of normal circadian rhythm is implicated in increasing the risk of developing cancers. In addition, deregulated expression of clock genes has been demonstrated in various types of cancer. These findings indicate a close association between circadian clock and cancer development and progression. Here, we review different evidences of this association in relation to molecular pathogenesis in gliomas.

Exploring the role of circadian clock gene and association with cancer pathophysiology

Most of the processes that occur in the mind and body follow natural rhythms. Those with a cycle length of about one day are called circadian rhythms. These rhythms are driven by a system of self-sustained clocks and are entrained by environmental cues such as light-dark cycles as well as food intake. In mammals, the circadian clock system is hierarchically organized such that the master clock in the suprachiasmatic nuclei of the hypothalamus integrates environmental information and synchronizes the phase of oscillators in peripheral tissues.The circadian system is responsible for regulating a variety of physiological and behavioral processes, including feeding behavior and energy metabolism. Studies revealed that the circadian clock system consists primarily of a set of clock genes. Several genes control the biological clock, including BMAL1, CLOCK (positive regulators), CRY1, CRY2, PER1, PER2, and PER3 (negative regulators) as indicators of the peripheral clock.Circadian has increasingly become an important area of medical research, with hundreds of studies pointing to the body's internal clocks as a factor in both health and disease. Thousands of biochemical processes from sleep and wakefulness to DNA repair are scheduled and dictated by these internal clocks. Cancer is an example of health problems where chronotherapy can be used to improve outcomes and deliver a higher quality of care to patients.In this article, we will discuss knowledge about molecular mechanisms of the circadian clock and the role of clocks in physiology and pathophysiology of concerns.

The NuA4 acetyltransferase and histone H4 acetylation promote replication recovery after topoisomerase I-poisoning

BACKGROUND

Histone acetylation plays an important role in DNA replication and repair because replicating chromatin is subject to dynamic changes in its structures. However, its precise mechanism remains elusive. In this report, we describe roles of the NuA4 acetyltransferase and histone H4 acetylation in replication fork protection in the fission yeast Schizosaccharomyces pombe.

RESULTS

Downregulation of NuA4 subunits renders cells highly sensitive to camptothecin, a compound that induces replication fork breakage. Defects in NuA4 function or mutations in histone H4 acetylation sites lead to impaired recovery of collapsed replication forks and elevated levels of Rad52 DNA repair foci, indicating the role of histone H4 acetylation in DNA replication and fork repair. We also show that Vid21 interacts with the Swi1-Swi3 replication fork protection complex and that Swi1 stabilizes Vid21 and promotes efficient histone H4 acetylation. Furthermore, our genetic analysis demonstrates that loss of Swi1 further sensitizes NuA4 and histone H4 mutant cells to replication fork breakage.

CONCLUSION

Considering that Swi1 plays a critical role in replication fork protection, our results indicate that NuA4 and histone H4 acetylation promote repair of broken DNA replication forks.

Tight Regulation of Srs2 Helicase Activity Is Crucial for Proper Functioning of DNA Repair Mechanisms

Proper DNA damage repair is one of the most vital and fundamental functions of every cell. Several different repair mechanisms exist to deal with various types of DNA damage, in various stages of the cell cycle and under different conditions. Homologous recombination is one of the most important repair mechanisms in all organisms. Srs2, a regulator of homologous recombination, is a DNA helicase involved in DNA repair, cell cycle progression and genome integrity. Srs2 can remove Rad51 from ssDNA, and is thought to inhibit unscheduled recombination. However, Srs2 has to be precisely regulated, as failure to do so is toxic and can lead to cell death. We noticed that a very slight elevation of the levels of Srs2 (by addition of a single extra copy of the SRS2 gene) leads to hyper-sensitivity of yeast cells to methyl methanesulfonate (MMS, a DNA damaging agent). This effect is seen in haploid, but not in diploid, cells. We analyzed the mechanism that controls haploid/diploid sensitivity and arrived to the conclusion that the sensitivity requires the activity of RAD59 and RDH54, whose expression in diploid cells is repressed. We carried out a mutational analysis of Srs2 to determine the regions of the protein required for the sensitization to genotoxins. Interestingly, Srs2 needs the HR machinery and its helicase activity for its toxicity, but does not need to dismantle Rad51. Our work underscores the tight regulation that is required on the levels of Srs2 activity, and the fact that Srs2 helicase activity plays a more central role in DNA repair than the ability of Srs2 to dismantle Rad51 filaments.

How long does telomerase extend telomeres? Regulation of telomerase release and telomere length homeostasis

Telomerase, the enzyme that replenishes telomeres, is essential for most eukaryotes to maintain their generations. Telomere length homeostasis is achieved via a balance between telomere lengthening by telomerase, and erosion over successive cell divisions. Impaired telomerase regulation leads to shortened telomeres and can cause defects in tissue maintenance. Telomeric DNA is composed of a repetitive sequence, which recruits the protective protein complex, shelterin. Shelterin, together with chromatin remodelling proteins, shapes the heterochromatic structure at the telomere and protects chromosome ends. Shelterin also provides a foothold for telomerase to be recruited and facilitates telomere extension. Such mechanisms of telomere recruitment and activation are conserved from unicellular eukaryotes to humans, with the rate of telomere extension playing an important role in determining the length maintained. Telomerase can be processive, adding multiple telomeric repeats before dissociating. However, a question remains: how does telomerase determine the number of repeats to add? In this review, I will discuss about how telomerase can monitor telomere extension using fission yeast as a model. I propose a model whereby the accumulation of the Pot1 complex on the synthesised telomere single-strand counteracts retention of telomerase via chromatin proteins and the similar system may be conserved in mammals.

Chromosome ends as adaptive beginnings: the potential role of dysfunctional telomeres in subtelomeric evolvability

Telomeres serve as protective caps that help the cell differentiate between the naturally occurring ends of chromosomes and double-stranded breaks. When telomere capping function becomes compromised, chromosome ends are subjected to elevated rates of chromosome alterations. These effects can be particularly dramatic in the telomere-adjacent subtelomeric region. While the catastrophic impact of severe telomere dysfunction on genome stability has been well documented, the adaptive telomere failure hypothesis considers an alternative role telomere dysfunction may play in adaptive evolution. This hypothesis suggests that low levels of telomere failure, induced by certain environmental stresses, can lead to elevated subtelomeric recombination. Mutational loss, duplication, or modification of subtelomeric contingency genes could ultimately facilitate adaptation by generating novel mutants better able to survive environmental stress. In this perspective, we discuss recent work that examined mild telomere dysfunction and its role in altering the adaptive potential of subtelomeric genes.

Induction and relocalization of telomeric repeat-containing RNAs during diauxic shift in budding yeast

Telomeres are maintained in a heterochromatic state that represses transcription of subtelomeric genes, a phenomenon known as telomere position effect. Nevertheless, telomeric DNA is actively transcribed, leading to the synthesis of telomeric repeat-containing noncoding RNA or TERRA. This nuclear noncoding RNA has been proposed to play important roles at telomeres, regulating their silencing, capping, repair and elongation by telomerase. In the budding yeast Saccharomyces cerevisiae, TERRA accumulation is repressed by telomeric silencing and the Rat1 exonuclease. On the other hand, telomere shortening promotes expression of TERRA. So far, little is known about the biological processes that induce TERRA expression in yeast. Understanding the dynamics of TERRA expression and localization is essential to define its function in telomere biology. Here, we aim to study the dynamics of TERRA expression during yeast cell growth. Using live-cell imaging, RNA-FISH and quantitative RT-PCR, we show that TERRA expression is induced as yeast cells undergo diauxic shift, a lag phase during which yeast cells switch their metabolism from anaerobic fermentation to oxidative respiration. This induction is transient as TERRA levels decrease during post-diauxic shift. The increased expression of TERRA is not due to the shortening of telomeres or increased stability of this transcript. Surprisingly, this induction is coincident with a cytoplasmic accumulation of TERRA molecules. Our results suggest that TERRA transcripts may play extranuclear functions with important implications in telomere biology and add a novel layer of complexity in the interplay between telomere biology, metabolism and stress response.

Rnr1's role in telomere elongation cannot be replaced by Rnr3: a role beyond dNTPs?

Telomeres, the nucleoprotein complexes at the end of eukaryotic chromosomes, protect them from degradation and ensure the replicative capacity of cells. In most human tumors and in budding yeast, telomere length is maintained by the activity of telomerase, an enzyme that adds dNTPs according to an internal RNA template. The dNTPs are generated with the help of the ribonucleotide reductase (RNR) complex. We have recently generated strains lacking the large subunit of RNR, Rnr1, which were kept viable by the expression of RNR complexes containing the Rnr1 homolog, Rnr3. Interestingly, we found that these Rnr1-deficient strains have short telomeres that are stably maintained, but cannot become efficiently elongated by telomerase. Thus, a basic maintenance of short telomeres is possible under conditions, where Rnr1 activity is absent, but a sustained elongation of short telomeres fully depends on Rnr1 activity. We show that Rnr3 cannot compensate for this telomeric function of Rnr1 even when overall cellular dNTP values are restored. This suggests that Rnr1 plays a role in telomere elongation beyond increasing cellular dNTP levels. Furthermore, our data indicate that telomerase may act in two different modes, one that is capable of coping with the "end-replication problem" and is functional even in the absence of Rnr1 and another required for the sustained elongation of short telomeres, which fully depends on the presence of Rnr1. Supply of dNTPs for telomere elongation is provided by the Mec1^ATR checkpoint, both during regular DNA replication and upon replication fork stalling. We discuss the implications of these results on telomere maintenance in yeast and cancer cells.

Mec1ATR is needed for extensive telomere elongation in response to ethanol in yeast

Telomere length homeostasis is essential for cell survival. In humans, telomeres shorten as a function of age. Short telomeres are known determinants of cell senescence and longevity. The yeast Saccharomyces cerevisiae expresses telomerase and maintains a strict telomere length homeostasis during vegetative growth. We have previously reported that different environmental signals promote changes in telomere length in S. cerevisiae. In particular, exposure to ethanol induces an extensive telomere elongation response due to a reduction in RAP1 mRNA and protein levels. Here we show that the reduction in Rap1 protein levels disrupts the physical interaction between Rap1 and Rif1, which in turn reduces the recruitment of these two proteins to telomeres during G2-phase. Although elongation of the shortest telomeres has been shown to depend on the Rif2 telomeric protein and on the Tel1(ATM) protein kinase, we show here that the extensive telomere elongation in response to ethanol exposure is Rif1 and Mec1 (ATR)-dependent. Our results fit a model in which Rif1 and Rap1 form a complex that is loaded onto telomeres at the end of S-phase. Reduced levels of the Rap1-Rif1 complex in ethanol lead to continuous telomere elongation in a Mec1-dependent process.

Psychoneuroimmunology-developments in stress research

Links between the central nervous stress system and peripheral immune cells in lymphoid organs have been detailed through 50 years of intensive research. The brain can interfere with the immune system, where chronic psychological stress inhibits many functions of the immune system. On the other hand, chronic peripheral inflammation-whether mild (during aging and psychological stress) or severe (chronic inflammatory diseases)-clearly interferes with brain function, leading to disease sequelae like fatigue but also to overt psychiatric illness. In recent years, it has been observed that psychological stress can be disease permissive, as in chronic inflammatory diseases, cancer, cardiovascular diseases, acute and chronic viral infections, sepsis, asthma, and others. We recognized that stress reactivity is programmed for a lifetime during a critical period between fetal life and early childhood, which then influences stress behavior and stress responses in adulthood. First phase II clinical studies, e.g., on cognitive behavioral therapy and mind-body therapies (e. g., mindfulness-based stress reduction), are available that show some benefits in stressful human diseases such as breast cancer and others. The field of psychoneuroimmunology has reached a firm ground and invites therapeutic approaches based on Good Clinical Practice phase III multicenter randomized controlled trials to influence stress responses and outcome in chronic illness.

Diverse roles of Dpb2, the non-catalytic subunit of DNA polymerase ε

Timely progression of living cells through the cell cycle is precisely regulated. This involves a series of phosphorylation events which are regulated by various cyclins, activated in coordination with the cell cycle progression. Phosphorylated proteins govern cell growth, division as well as duplication of the genetic material and transcriptional activation of genes involved in these processes. A subset of these tightly regulated genes, which depend on the MBF transcription factor and are mainly involved in DNA replication and cell division, is transiently activated at the transition from G1 to S phase. A Saccharomyces cerevisiae mutant in the Dpb2 non-catalytic subunit of DNA polymerase ε (Polε) demonstrates abnormalities in transcription of MBF-dependent genes even in normal growth conditions. It is, therefore, tempting to speculate that Dpb2 which, as described previously, participates in the early stages of DNA replication initiation, has an impact on the regulation of replication-related genes expression with possible implications for genomic stability.

Regulation of DNA Replication through Natural Impediments in the Eukaryotic Genome

All living organisms need to duplicate their genetic information while protecting it from unwanted mutations, which can lead to genetic disorders and cancer development. Inaccuracies during DNA replication are the major cause of genomic instability, as replication forks are prone to stalling and collapse, resulting in DNA damage. The presence of exogenous DNA damaging agents as well as endogenous difficult-to-replicate DNA regions containing DNA–protein complexes, repetitive DNA, secondary DNA structures, or transcribing RNA polymerases, increases the risk of genomic instability and thus threatens cell survival. Therefore, understanding the cellular mechanisms required to preserve the genetic information during S phase is of paramount importance. In this review, we will discuss our current understanding of how cells cope with these natural impediments in order to prevent DNA damage and genomic instability during DNA replication.

Telomere Length Assessment for Prediction of Organ Transplantation Outcome. Future or Failure: A Review of the Literature

Telomeres are located at each end of eukaryotic chromosomes. Their functional role is genomic stability maintenance. The protective role of telomeres depends on various factors, including number of nucleotides repeats, telomere-binding proteins, and telomerase activity. Organ transplantation is the preferred replacement therapy in the case of chronic kidney disease and the only possibility of sustaining recipients’ life in the case of advanced liver failure. While the prevalence of acute rejection is constantly decreasing, prevention of transplanted organ long-term function loss is still challenging. It has been demonstrated that post-transplant stressors accelerate aging of the allografts manifested through telomere shortening.

The aim of this paper was to evaluate the importance of telomere length assessment for prediction of organ transplantation outcome. Literature review included the 10 most important studies regarding linkage between allograft function and telomere erosion, including 2 of our own reports.

Telomere length assessment is useful to predict organ transplantation outcome. The importance of telomere length as a prediction marker depends on the analyzed material. To obtain reliable results, both graft cells (donor material) and lymphocytes (recipient material) should be examined. In the case of kidney transplantation, assessment of telomere length in the early post-transplant period allows prediction of the long-term function of the transplanted organ. To increase the accuracy of transplantation outcome prediction, telomere length assessment should be combined with evaluation of other aging biomarkers, like CDKN2A (p16). Large-scale clinical studies regarding telomere length measurement, including genome wide association analysis introducing relevant genetic factors, are needed for the future.