Telomere DNA length regulation is influenced by seasonal temperature differences in short-lived but not in long-lived reef-building corals.

Telomeres are environment-sensitive regulators of health and aging. Here,we present telomere DNA length analysis of two reef-building coral genera revealing that the long- and short-term water thermal regime is a key driver of between-colony variation across the Pacific Ocean. Notably, there are differences between the two studied genera. The telomere DNA lengths of the short-lived, more stress-sensitive Pocillopora spp. colonies were largely determined by seasonal temperature variation, whereas those of the long-lived, more stress-resistant Porites spp. colonies were insensitive to seasonal patterns, but rather influenced by past thermal anomalies. These results reveal marked differences in telomere DNA length regulation between two evolutionary distant coral genera exhibiting specific life-history traits. We propose that environmentally regulated mechanisms of telomere maintenance are linked to organismal performances, a matter of paramount importance considering the effects of climate change on health.

Longevity strategies in response to light in the reef coral Stylophora pistillata.

Aging is a multifactorial process that results in progressive loss of regenerative capacity and tissue function while simultaneously favoring the development of a large array of age-related diseases. Evidence suggests that the accumulation of senescent cells in tissue promotes both normal and pathological aging. Oxic stress is a key driver of cellular senescence. Because symbiotic long-lived reef corals experience daily hyperoxic and hypoxic transitions, we hypothesized that these long-lived animals have developed specific longevity strategies in response to light. We analyzed transcriptome variation in the reef coral Stylophora pistillata during the day-night cycle and revealed a signature of the FoxO longevity pathway. We confirmed this pathway by immunofluorescence using antibodies against coral FoxO to demonstrate its nuclear translocation. Through qPCR analysis of nycthemeral variations of candidate genes under different light regimens, we found that, among genes that were specifically up- or downregulated upon exposure to light, human orthologs of two "light-up" genes (HEY1 and LONF3) exhibited anti-senescence properties in primary human fibroblasts. Therefore, these genes are interesting candidates for counteracting skin aging. We propose a large screen for other light-up genes and an investigation of the biological response of reef corals to light (e.g., metabolic switching) to elucidate these processes and identify effective interventions for promoting healthy aging in humans.

Tissue-Specific Suppression of Thyroid Hormone Signaling in Various Mouse Models of Aging.

DNA damage contributes to the process of aging, as underscored by premature aging syndromes caused by defective DNA repair. Thyroid state changes during aging, but underlying mechanisms remain elusive. Since thyroid hormone (TH) is a key regulator of metabolism, changes in TH signaling have widespread effects. Here, we reveal a significant common transcriptomic signature in livers from hypothyroid mice, DNA repair-deficient mice with severe (Csbm/m/Xpa-/-) or intermediate (Ercc1-/Δ-7) progeria and naturally aged mice. A strong induction of TH-inactivating deiodinase D3 and decrease of TH-activating D1 activities are observed in Csbm/m/Xpa-/- livers. Similar findings are noticed in Ercc1-/Δ-7, in naturally aged animals and in wild-type mice exposed to a chronic subtoxic dose of DNA-damaging agents. In contrast, TH signaling in muscle, heart and brain appears unaltered. These data show a strong suppression of TH signaling in specific peripheral organs in premature and normal aging, probably lowering metabolism, while other tissues appear to preserve metabolism. D3-mediated TH inactivation is unexpected, given its expression mainly in fetal tissues. Our studies highlight the importance of DNA damage as the underlying mechanism of changes in thyroid state. Tissue-specific regulation of deiodinase activities, ensuring diminished TH signaling, may contribute importantly to the protective metabolic response in aging.

Acacetin and chrysin, two polyphenolic compounds, alleviate telomeric position effect in human cells.

We took advantage of the ability of human telomeres to silence neighboring genes (telomere position effect or TPE) to design a high-throughput screening assay for drugs altering telomeres. We identified, for the first time, that two dietary flavones, acacetin and chrysin, are able to specifically alleviate TPE in human cells. We further investigated their influence on telomere integrity and showed that both drugs drastically deprotect telomeres against DNA damage response. However, telomere deprotection triggered by shelterin dysfunction does not affect TPE, indicating that acacetin and chrysin target several functions of telomeres. These results show that TPE-based screening assays represent valuable methods to discover new compounds targeting telomeres.Molecular Therapy-Nucleic Acids (2013) 2, e116; doi:10.1038/mtna.2013.42; published online 20 August 2013.

Replication timing of human telomeres is chromosome arm-specific, influenced by subtelomeric structures and connected to nuclear localization.

The mechanisms governing telomere replication in humans are still poorly understood. To fill this gap, we investigated the timing of replication of single telomeres in human cells. Using in situ hybridization techniques, we have found that specific telomeres have preferential time windows for replication during the S-phase and that these intervals do not depend upon telomere length and are largely conserved between homologous chromosomes and between individuals, even in the presence of large subtelomeric segmental polymorphisms. Importantly, we show that one copy of the 3.3 kb macrosatellite repeat D4Z4, present in the subtelomeric region of the late replicating 4q35 telomere, is sufficient to confer both a more peripheral localization and a later-replicating property to a de novo formed telomere. Also, the presence of beta-satellite repeats next to a newly created telomere is sufficient to delay its replication timing. Remarkably, several native, non-D4Z4-associated, late-replicating telomeres show a preferential localization toward the nuclear periphery, while several early-replicating telomeres are associated with the inner nuclear volume. We propose that, in humans, chromosome arm-specific subtelomeric sequences may influence both the spatial distribution of telomeres in the nucleus and their replication timing.

D4Z4 as a prototype of CTCF and lamins-dependent insulator in human cells.

Using cellular models that mimic the organizations of the subtelomeric 4q35 locus found in patients affected with Facio-Scapulo-Humeral Dystrophy (FSHD) and in healthy individuals, we recently investigated the biological function of the D4Z4 macrosatellite in this subtelomeric context.We demonstrated that D4Z4 acts as a CTCF and A-type lamins dependent insulator element exhibiting both enhancer- blocking and barrier activities, and displaces a telomere towards the nuclear periphery. This peripheral positioning activity lies within a short sequence that interacts with CTCF and A-type lamins. Depletion in either of these two proteins suppresses these perinuclear activities, revealing the existence of a subtelomeric sequence that is sufficient to position an adjacent telomere to the nuclear periphery. We discuss here the biological implications of these results in the light of our current knowledge in related fields and the potential implication of other CTCF and A-type lamins insulators in the light of human pathologies.

Identification of a perinuclear positioning element in human subtelomeres that requires A-type lamins and CTCF.

The localization of genes within the nuclear space is of paramount importance for proper genome functions. However, very little is known on the cis-acting elements determining subnuclear positioning of chromosome segments. We show here that the D4Z4 human subtelomeric repeat localizes a telomere at the nuclear periphery. This perinuclear activity lies within an 80 bp sequence included within a region known to interact with CTCF and A-type Lamins. We further show that a reduced level of either CTCF or A-type Lamins suppresses the perinuclear activities of D4Z4 and that an array of multimerized D4Z4 sequence, which has lost its ability to bind CTCF and A-type Lamins, is not localized at the periphery. Overall, these findings reveal the existence of an 80 bp D4Z4 sequence that is sufficient to position an adjacent telomere to the nuclear periphery in a CTCF and A-type lamins-dependent manner. Strikingly, this sequence includes a 30 bp GA-rich motif, which binds CTCF and is present at several locations in the human genome.

The D4Z4 macrosatellite repeat acts as a CTCF and A-type lamins-dependent insulator in facio-scapulo-humeral dystrophy.

Both genetic and epigenetic alterations contribute to Facio-Scapulo-Humeral Dystrophy (FSHD), which is linked to the shortening of the array of D4Z4 repeats at the 4q35 locus. The consequence of this rearrangement remains enigmatic, but deletion of this 3.3-kb macrosatellite element might affect the expression of the FSHD-associated gene(s) through position effect mechanisms. We investigated this hypothesis by creating a large collection of constructs carrying 1 to >11 D4Z4 repeats integrated into the human genome, either at random sites or proximal to a telomere, mimicking thereby the organization of the 4q35 locus. We show that D4Z4 acts as an insulator that interferes with enhancer-promoter communication and protects transgenes from position effect. This last property depends on both CTCF and A-type Lamins. We further demonstrate that both anti-silencing activity of D4Z4 and CTCF binding are lost upon multimerization of the repeat in cells from FSHD patients compared to control myoblasts from healthy individuals, suggesting that FSHD corresponds to a gain-of-function of CTCF at the residual D4Z4 repeats. We propose that contraction of the D4Z4 array contributes to FSHD physio-pathology by acting as a CTCF-dependent insulator in patients.

Telomeric position effect: from the yeast paradigm to human pathologies?

Alteration of the epigenome is associated with a wide range of human diseases. Therefore, deciphering the pathways that regulate the epigenetic modulation of gene expression is a major milestone for the understanding of diverse biological mechanisms and subsequently human pathologies. Although often evoked, little is known on the implication of telomeric position effect, a silencing mechanism combining telomere architecture and classical heterochromatin features, in human cells. Nevertheless, this particular silencing mechanism has been investigated in different organisms and several ingredients are likely conserved during evolution. Subtelomeres are highly dynamic regions near the end of the chromosomes that are prone to recombination and may buffer or facilitate the spreading of silencing that emanates from the telomere. Therefore, the composition and integrity of these regions also concur to the propensity of telomeres to regulate the expression, replication and recombination of adjacent regions. Here we describe the similarities and disparities that exist among the different species at chromosome ends with regard to telomeric silencing regulation with a special accent on its implication in numerous human pathologies.

Interactions of cryptolepine and neocryptolepine with unusual DNA structures.

Cryptolepine, the main alkaloid present in the roots of Cryptolepis sanguinolenta, presents a large spectrum of biological properties. It has been reported to behave like a DNA intercalator with a preference for GC-rich sequences. In this study, dialysis competition assay and mass spectrometry experiments were used to determine the affinity of cryptolepine and neocryptolepine for DNA structures among duplexes, triplexes, quadruplexes and single strands. Our data confirm that cryptolepine and neocryptolepine prefer GC over AT-rich duplex sequences, but also recognize triplex and quadruplex structures. These compounds are weak telomerase inhibitors and exhibit a significant preference for triplexes over quadruplexes or duplexes.