Early Embryo Development: What Does Daddy Do?

Infertility represents a major global health challenge, with male infertility accounting for a significant proportion of cases, yet its underlying causes remain elusive in many instances. Traditionally, spermatozoa were viewed merely as DNA carriers, with little consideration given to their role beyond fertilization. Recent research, however, is challenging this view, revealing that spermatozoa are far more than passive delivery vehicles. They carry a complex array of molecules, particularly RNAs, which actively influence fertilization, early embryo development, and the transmission of paternal traits. These sperm-carried RNAs, including mRNAs, small RNAs, and noncoding RNAs, regulate gene expression in both spermatozoa and embryo, with profound implications for offspring development. Additionally, environmental factors, such as lifestyle choices and exposure to toxins, have been shown to affect sperm RNA composition, highlighting the dynamic interplay between genetics and the environment in shaping fertility. This emerging and evolving understanding of sperm function challenges traditional reproductive biology and offers new insights into male infertility, particularly in cases that remain unexplained by current diagnostic methods. Although the exact molecular mechanisms underlying these processes are still being investigated, this paradigm shift opens the door to innovative diagnostic tools and therapeutic strategies for treating male infertility. By uncovering the critical role of sperm RNAs, these findings not only enhance our understanding of reproductive biology but also hold the promise to improve assisted reproductive technologies and outcomes for infertile couples.

Human sperm RNA in male infertility

The function and value of specific sperm RNAs in apparently idiopathic male infertility are currently poorly understood. Whether differences exist in the sperm RNA profile between patients with infertility and fertile men needs clarification. Similarly, the utility of sperm RNAs in predicting successful sperm retrieval and assisted reproductive technique (ART) outcome is unknown. Patients with infertility and fertile individuals seem to have differences in the expression of non-coding RNAs that regulate genes controlling spermatogenesis. Several RNAs seem to influence embryo quality and development. Also, RNA types seem to predict successful sperm retrieval in patients with azoospermia. These findings suggest that sperm RNAs could influence decision-making during the management of patients with infertility. This evidence might help to identify possible therapeutic approaches aimed at modulating the expression of dysregulated genes in patients with infertility. Performing prospective studies with large sample sizes is necessary to investigate cost-effective panels consisting of proven molecular targets to ensure that this evidence can be translated to clinical practice.

Telomeric RNAs, TERRA, as a Potential Biomarker for Spermatozoa Quality

Characterization of long non-coding telomeric repeat-containing RNAs in sperm of normozoospermic and oligoasthenozoospermic men as new biomarker of idiopathic male infertility. We conducted an observational prospective study with two groups of men with normal or orligoasthenozoospermic spermiogram, aged 40 and above. Fertility parameters were analyzed in men undergoing intracytoplasmic sperm injection with donor oocytes, to avoid the female factor. Telomeric RNAs and telomere length were measured by quantitative fluorescent in situ hybridization. Data from seminal parameters and in-vitro fertilization were assessed according to IVIRMA protocols. Patients with oligoasthenozoospermia, who had worse seminal parameters, also obtained embryos with lower inner-cell-mass quality (p = 0.04), despite using donor oocytes. While mean levels of telomeric RNAs were similar for both groups, the percentage of spermatozoa with more than 3 foci was higher in oligoasthenozoospermic men (p = 0.02). Regarding telomere length, oligoasthenozoospermic men had shorter mean, a higher accumulation of short telomeres (15th percentile; p = 0.03) and a lower percentage of very-long telomeres (85th percentile; p = 0.01). Finally, a positive correlation was found between telomeric-RNAs intensity and total progressive motility in the spermatozoa of normozoospermic patients (r = 0.5; p = 0.03). Telomeric parameters were altered in the spermatozoa of the oligoasthenozoospermic group, which also showed lower quality embryos. Interestingly, in the normozoospermic group, a correlation was found between progressive motility and telomeric RNA levels, suggesting that they could be a good biomarker of sperm quality. Further studies are required to confirm these results and translate them into the clinical practice.Trial registration number: 1711-MAD-109-CB, 07/07/2021.

Quantifying telomere transcripts as tool to improve risk assessment for genetic instability and genotoxicity

Telomere repeat-containing RNAs (TERRA) are transcribed from telomeres as long non-coding RNAs and are part of the telomere structure with protective function. The genetic stability of cells requires telomeric repeats at the ends of chromosomes. Maintenance of telomere length (TL) is essential for proliferative capacity and chromosomal integrity. In contrast, telomere shortening is a recognized risk factor for carcinogenesis and a biomarker of aging due to the cumulative effects of environmental exposures and life experiences such as trauma or stress. In this context, telomere repeats are lost due to cell proliferation, but are also susceptible to stress factors including reactive oxygen species (ROS) inducing oxidative base damage. Quantitative PCR (qPCR) of genomic DNA is an established method to analyze TL as a tool to detect genotoxic events. That same qPCR method can be applied to RNA converted into cDNA to quantify TERRA as a useful tool to perform high-throughput screenings. This short review summarizes relevant qPCR studies using both TL and TERRA quantification, provides an overall view of the molecular mechanisms of telomere protection against ROS by TERRA, and summarizes the presented studies comparing the results at DNA and RNA levels, which indicate that fluctuations at transcript level might reflect a short-term response. Therefore, we conclude that performing both of these measurements together will improve genotoxicity studies.

TERRA beyond cancer: the biology of telomeric repeat-containing RNAs in somatic and germ cells

The telomeric noncoding RNA TERRA is a key component of telomeres and it is widely expressed in normal as well as cancer cells. In the last 15 years, several publications have shed light on the role of TERRA in telomere homeostasis and cell survival in cancer cells. However, only few studies have investigated the regulation or the functions of TERRA in normal tissues. A better understanding of the biology of TERRA in non-cancer cells may provide unexpected insights into how these lncRNAs are transcribed and operate in cells, and their potential role in physiological processes, such as aging, age-related pathologies, inflammatory processes and human genetic diseases. In this review we aim to discuss the findings that have advanced our understanding of the biology of TERRA using non-cancer mammalian cells as a model system.

A pilot study of LINE-1 copy number and telomere length with aging in human sperm

PURPOSE

Unlike other cells in the body, in sperm, telomere length (TL) increases with age. TL can regulate nearby genes, and the subtelomeric region is rich in retrotransposons. We hypothesized that age-related telomere lengthening in sperm might suppress Long Interspersed Element 1 (LINE-1/L1), the only competent retrotransposon in humans.

METHODS

We measured L1 copy number (L1-CN) and sperm telomere length (STL) from young and older men to evaluate the relationship between age, TL and L1-CN. We also evaluated L1-CN and TL in individual sperm to determine whether these variables influence sperm morphology. STL was assayed by Multiplex quantitative polymerase chain reaction method (mmqPCR) and L1-CN by Quantitative polymerase chain reaction (qPCR).

RESULTS

We found that STL increased, and L1-CN decreased significantly with paternal age. STL in normal single sperm was significantly higher than in abnormal sperm. L1-CN did not differ between normal and abnormal sperm. Furthermore, morphologically normal sperm have longer telomeres than abnormal sperm.

CONCLUSIONS

Elongation of telomeres in the male germline could repress retrotransposition, which tends to increase with cellular aging. More studies in larger cohorts across a wide age span are needed to confirm our conclusions and explore their biological and clinical significance.

Cohesin SMC1β promotes closed chromatin and controls TERRA expression at spermatocyte telomeres

Previous data showed that meiotic cohesin SMC1β protects spermatocyte telomeres from damage. The underlying reason, however, remained unknown as the expressions of telomerase and shelterin components were normal in Smc1β -/- spermatocytes. Here. we report that SMC1β restricts expression of the long noncoding RNA TERRA (telomeric repeat containing RNA) in spermatocytes. In somatic cell lines increased TERRA was reported to cause telomere damage through altering telomere chromatin structure. In Smc1β -/- spermatocytes, we observed strongly increased levels of TERRA which accumulate on damaged chromosomal ends, where enhanced R-loop formation was found. This suggested a more open chromatin configuration near telomeres in Smc1β -/- spermatocytes, which was confirmed by ATAC-seq. Telomere-distal regions were not affected by the absence of SMC1β but RNA-seq revealed increased transcriptional activity in telomere-proximal regions. Thus, SMC1β promotes closed chromatin specifically near telomeres and limits TERRA expression in spermatocytes.

Paternal aging impacts mitochondrial DNA content and telomere length in mouse embryos

Mitochondrial DNA (mtDNA) copy number and telomere length (TL) in blastocysts derived from the same male mice at young (10-19-week-old) and aged (40-49-week-old) time points and mtDNA and TL in the hearts of offspring derived from young and aged male mice were examined. Paternal aging correlated with reduced mtDNA and TL in blastocysts. mtDNA and TL were significantly correlated, which was also observed in bovine blastocysts. Moreover, mtDNA in the heart of offspring was reduced in male mice with paternal aging. In conclusion, paternal aging affects embryonic mtDNA and TL, potentially impacting their offspring.

Strategies for meiotic sex chromosome dynamics and telomeric elongation in Marsupials

During meiotic prophase I, homologous chromosomes pair, synapse and recombine in a tightly regulated process that ensures the generation of genetically variable haploid gametes. Although the mechanisms underlying meiotic cell division have been well studied in model species, our understanding of the dynamics of meiotic prophase I in non-traditional model mammals remains in its infancy. Here, we reveal key meiotic features in previously uncharacterised marsupial species (the tammar wallaby and the fat-tailed dunnart), plus the fat-tailed mouse opossum, with a focus on sex chromosome pairing strategies, recombination and meiotic telomere homeostasis. We uncovered differences between phylogroups with important functional and evolutionary implications. First, sex chromosomes, which lack a pseudo-autosomal region in marsupials, had species specific pairing and silencing strategies, with implications for sex chromosome evolution. Second, we detected two waves of γH2AX accumulation during prophase I. The first wave was accompanied by low γH2AX levels on autosomes, which correlated with the low recombination rates that distinguish marsupials from eutherian mammals. In the second wave, γH2AX was restricted to sex chromosomes in all three species, which correlated with transcription from the X in tammar wallaby. This suggests non-canonical functions of γH2AX on meiotic sex chromosomes. Finally, we uncover evidence for telomere elongation in primary spermatocytes of the fat-tailed dunnart, a unique strategy within mammals. Our results provide new insights into meiotic progression and telomere homeostasis in marsupials, highlighting the importance of capturing the diversity of meiotic strategies within mammals.

The generation of haploid gametes is a hallmark of sexual reproduction. And this is accomplished by a complex, albeit tightly regulated, reductional cell division called meiosis. Although meiosis has been extensively studied in eutherian mammal model species, our understanding of the mechanisms regulating chromosome synapsis, recombination and segregation during meiosis progression is still incomplete especially in non-eutherian mammals. To fill this gap and capture the diversity of meiotic strategies among mammals, we study previously uncharacterised representative marsupial species, an evolutionary assemblage that last shared a common ancestry more than 80 million years ago. We uncover novel, hence non-canonical, strategies for sex chromosome pairing, DNA repair, recombination and transcription. Most importantly, we reveal the uniqueness of marsupial meiosis, which includes the unprecedented detection of alternative mechanism (ALT) for the paternal control of telomere length during prophase I. Our findings suggest that ALT (previously only associated to cancer cells) could play a role in telomere homeostasis in mammalian germ cells.

Insights into the Role of Telomeres in Human Embryological Parameters. Opinions Regarding IVF

Telomeres promote genome integrity by protecting chromosome ends from the activation of the DNA damage response and protecting chromosomes from the loss of coding sequences due to the end replication problem. Telomere length (TL) is progressively shortened as age progresses, thus resulting in cellular senescence. Therefore, TL is in strong adverse linear correlation with aging. Mounting evidence supports the notion that telomeres and male/female infertility are in a close relationship, posing the biology of telomeres as a hot topic in the era of human-assisted reproduction. Specifically, the length of sperm telomeres is gradually increasing as men get older, while the telomere length of the oocytes seems not to follow similar patterns with that of sperm. Nonetheless, the telomere length of the embryos during the cleavage stages seems to have a paternal origin, but the telomere length can be further extended by telomerase activity during the blastocyst stage. The latter has been proposed as a new molecular biomarker with strong predictive value regarding male infertility. As far as the role of telomeres in assisted reproduction, the data is limited but the length of telomeres in both gametes seems to be affected mainly by the cause of infertility rather than the assisted reproductive therapy (ART) procedure itself. The present review aims to shed more light into the role of telomeres in human embryological parameters, including gametes and embryos and also presents opinions regarding the association between telomeres and in vitro fertilization (IVF).

TERRA: A Novel Biomarker of Embryo Quality and Art Outcome

Telomeres are considered to be an internal biological clock, and their progressive shortening has been associated with the risk of age-related diseases and reproductive alterations. Over recent years, an increasing number of studies have focused on the association between telomere length and fertility, identifying sperm telomere length (STL) as a novel biomarker of male fertility. Although typically considered to be repeated DNA sequences, telomeres have recently been shown to also include a long non-coding RNA (lncRNA) known as TERRA (telomeric repeat-containing RNAs). Interestingly, males with idiopathic infertility show reduced testicular TERRA expression, suggesting a link between TERRA and male fertility. The aim of this study was to investigate the role of seminal TERRA expression in embryo quality. To this end, STL and TERRA expression were quantified by Real Time qPCR in the semen of 35 men who underwent assisted reproductive technologies (ART) and 30 fertile men. We found that TERRA expression in semen and STL was reduced in patients that underwent ART (both p < 0.001). Interestingly, TERRA and STL expressions were positively correlated (p = 0.010), and TERRA expression was positively associated with embryo quality (p < 0.001). These preliminary findings suggest a role for TERRA in the maintenance of sperm telomere integrity during gametogenesis, and for the first time, TERRA expression was found as a predictive factor for embryo quality in the setting of assisted reproduction.

The spatiotemporal expression of TERT and telomere repeat binding proteins in the postnatal mouse testes

Telomeres consist of repetitive DNA sequences and telomere-associated proteins. Telomeres located at the ends of eukaryotic chromosomes undergo shortening due to DNA replication, genotoxic factors and reactive oxygen species. The short telomeres are elongated by the enzyme telomerase expressed in the germ line, embryonic and stem cells. Telomerase is in the structure of ribonucleoprotein composed of telomerase reverse transcriptase (TERT), telomerase RNA component (Terc) and other components. Among telomere-associated proteins, telomeric repeat binding factor 1 (TRF1) and 2 (TRF2) exclusively bind to the double-stranded telomeric DNA to regulate its length. However, protection of telomeres 1 (POT1) interacts with the single-stranded telomeric DNA to protect from DNA damage response. Herein, we characterised the spatial and temporal expression of the TERT, TRF1, TRF2 and POT1 proteins in the postnatal mouse testes at the ages of 6, 8, 16, 20, 29, 32 and 88 days by using immunohistochemistry. Significant differences in the spatiotemporal expression patterns and levels of these proteins were determined in the postnatal testes (p < .05). These findings indicate that TERT and telomere repeat binding proteins seem to be required for maintaining the length and structural integrity of telomeres in the spermatogenic cells from newborn to adult terms.

Decreased expression of TERT and telomeric proteins as human ovaries age may cause telomere shortening

OBJECTIVE

Telomeres are repetitive sequences localized at the ends of eukaryotic chromosomes comprising noncoding DNA and telomere-binding proteins. TRF1 and TRF2 both bind to the double-stranded telomeric DNA to regulate its length throughout the lifespan of eukaryotic cells. POT1 interacts with single-stranded telomeric DNA and contributes to protecting genomic integrity. Previous studies have shown that telomeres gradually shorten as ovaries age, coinciding with fertility loss. However, the molecular background of telomere shortening with ovarian aging is not fully understood.

METHODS

The present study aimed to determine the spatial and temporal expression levels of the TERT, TRF1, TRF2, and POT1 proteins in different groups of human ovaries: fetal (n = 11), early postnatal (n = 10), premenopausal (n = 12), and postmenopausal (n = 14). Also, the relative telomere signal intensity of each group was measured using the Q-FISH method.

RESULTS

We found that the telomere signal intensities decreased evenly and significantly from fetal to postmenopausal groups (P < 0.05). The TERT, TRF1, TRF2, and POT1 proteins were localized in the cytoplasmic and nuclear regions of the oocytes, granulosa and stromal cells. Furthermore, the expression levels of these proteins reduced significantly from fetal to postmenopausal groups (P < 0.05).

CONCLUSION

These findings suggest that decreased TERT and telomere-binding protein expression may underlie the telomere shortening of ovaries with age, which may be associated with female fertility loss. Further investigations are required to elicit the molecular mechanisms regulating the gradual decrease in the expression of TERT and telomere-binding proteins in human oocytes and granulosa cells during ovarian aging.

Long non-coding RNAs: novel players in the pathogenesis of polycystic ovary syndrome

Long non-coding RNAs (lncRNAs) are a class of transcripts (>200 nucleotides) lacking protein-coding capacity. Based on the complex three-dimensional structure, lncRNAs are involved in many biological processes and can regulate the expression of target genes at chromatin modification, transcriptional and post-transcriptional levels. LncRNAs have been studied in multiple diseases but little is known about their role(s) in polycystic ovary syndrome (PCOS), the most common endocrinological disorder in reproductive-aged women around the world. In this review, we characterized and explored the potential mechanisms of lncRNAs in the pathogenesis of PCOS. We found that lncRNAs play a molecular role in PCOS mainly by functioning as the competitive endogenous RNA (ceRNA) and are significantly correlated with some clinical phenotypes. We summarized in detail regarding aberrant lncRNAs in different specimens of women with PCOS [i.e., granulosa cells (GCs), cumulus cells (CCs), follicular fluid (FF), peripheral blood] and various PCOS rodent models [i.e., dehydroepiandrosterone (DHEA) and letrozole induced models]. In clinical practice, detection of lncRNAs in serum might enable early diagnosis. Furthermore, new lncRNA-based classifications might be emerging as potent predictors of a particular phenotype in PCOS. Overall, we proposed new insights for the application of precision medicine approaches to the management of PCOS.

Dynamics of telomeric repeat-containing RNA expression in early embryonic cleavage stages with regards to maternal age

Telomeres are transcribed into long non-coding RNAs known as Telomeric Repeat-Containing RNA (TERRA). They have been shown to be essential regulators of telomeres and to act as epigenomic modulators at extra-telomeric sites. However the role of TERRA during early embryonic development has never been investigated. Here, we show that TERRA is expressed in murine and bovine early development following a wave pattern. It starts at 4-cell stage, reaching a maximum at the 16-cell followed by a decline at the morula and blastocyst stages. Moreover, TERRA expression is not affected by increasing oocyte donor age whereas telomere length does. This indicates that TERRA expression is independent of the telomere length in early development. Our findings anticipate an essential role of TERRA in early stages of development and this might be useful in the future for a better understanding of age related female infertility.

Characterization of telomeric repeat-containing RNA (TERRA) localization and protein interactions in primordial germ cells of the mouse†

Telomeres are dynamic nucleoprotein structures capping the physical ends of linear eukaryotic chromosomes. They consist of telomeric DNA repeats (TTAGGG), the shelterin protein complex, and telomeric repeat-containing RNA (TERRA). Proposed TERRA functions are wide ranging and include telomere maintenance, telomerase inhibition, genomic stability, and alternative lengthening of telomere. However, the presence and role of TERRA in primordial germ cells (PGCs), the embryonic precursors of germ cells, is unknown. Using RNA-fluorescence in situ hybridization, we identify TERRA transcripts in female PGCs at 11.5, 12.5, and 13.5 days postcoitum. In male PGCs, the earliest detection TERRA was at 12.5 dpc where we observed cells with either zero or one TERRA focus. Using qRT-PCR, we evaluated chromosome-specific TERRA expression. Female PGCs showed TERRA expression at 11.5 dpc from eight different chromosome subtelomeric regions (chromosomes 1, 2, 7, 9, 11, 13, 17, and 18) while in male PGCs, TERRA expression was confined to the chromosome 17. Most TERRA transcription in 13.5 dpc male PGCs arose from chromosomes 2 and 6. TERRA interacting proteins were evaluated using identification of direct RNA interacting proteins (iDRiP), which identified 48 in female and 26 in male protein interactors from PGCs at 13.5 dpc. We validated two different proteins: the splicing factor, proline- and glutamine-rich (SFPQ) in PGCs and non-POU domain-containing octamer-binding protein (NONO) in somatic cells. Taken together, our data indicate that TERRA expression and interactome during PGC development are regulated in a dynamic fashion that is dependent on gestational age and sex.

Telomere length: lights and shadows on their role in human reproduction

Telomeres are repeated DNA sequences whose main function is to preserve genome stability, protecting chromosomes ends from shortening caused by progressive loss during each cell replication or DNA damage. Telomere length regulation is normally achieved by telomerase enzyme, whose activity is progressively shut off during embryonic differentiation in somatic tissues, whereas it is maintained in germ cells, activated lymphocytes, and certain types of stem cell populations. The maintenance of telomerase activity for a longer time is necessary for germ cells to delay telomere erosion, thus avoiding chromosome segregation defects that could contribute to aneuploid or unbalanced gametes. Over the last few years, telomere biology has become an important topic in the field of human reproduction, encouraging several studies to focus on the relation between telomere length and spermatogenesis and male fertility, embryo development and quality during assisted reproductive treatment, and female pathologies as polycystic ovary, premature ovarian insufficiency, and endometriosis. This review analyzes whether telomere length in germ cells is related to reproduction fitness, whether telomere length is related to pathologies associated with male and female fertility, and whether measurement of telomere length could represent a biomarker of germ cell and embryo quality. Telomere length could be considered a molecular marker of spermatogenesis and sperm quality and is somewhat related to male fertility potential. Fewer evidence, although promising, is available for oocytes, female (in)fertility, and embryo quality. The increasing evidence for a role of telomeres and telomere length in human reproduction, indeed, has expanded the historical view of considering them just a marker of aging. Telomere length might have in the future a prognostic potential in couple infertility, especially useful to select best germ cells with the greatest potential of fertilization.

Telomere Dynamics Throughout Spermatogenesis

Telomeres are repeat regions of DNA that cap either end of each chromosome, thereby providing stability and protection from the degradation of gene-rich regions. Each cell replication causes the loss of telomeric repeats due to incomplete DNA replication, though it is well-established that progressive telomere shortening is evaded in male germ cells by the maintenance of active telomerase. However, germ cell telomeres are still susceptible to disruption or insult by oxidative stress, toxicant exposure, and aging. Our aim was to examine the relative telomere length (rTL) in an outbred Sprague Dawley (SD) and an inbred Brown Norway (BN) rat model for paternal aging. No significant differences were found when comparing pachytene spermatocytes (PS), round spermatids (RS), and sperm obtained from the caput and cauda of the epididymis of young and aged SD rats; this is likely due to the high variance observed among individuals. A significant age-dependent decrease in rTL was observed from 115.6 (±6.5) to 93.3 (±6.3) in caput sperm and from 142.4 (±14.6) to 105.3 (±2.5) in cauda sperm from BN rats. Additionally, an increase in rTL during epididymal maturation was observed in both strains, most strikingly from 115.6 (±6.5) to 142 (±14.6) in young BN rats. These results confirm the decrease in rTL in rodents, but only when an inbred strain is used, and represent the first demonstration that rTL changes as sperm transit through the epididymis.

Telomeres, Telomerase and Ageing

Telomeres are specialised structures at the end of linear chromosomes. They consist of tandem repeats of the hexanucleotide sequence TTAGGG, as well as a protein complex called shelterin. Together, they form a protective loop structure against chromosome fusion and degradation. Shortening or damage to telomeres and opening of the loop induce an uncapped state that triggers a DNA damage response resulting in senescence or apoptosis.Average telomere length, usually measured in human blood lymphocytes, was thought to be a biomarker for ageing, survival and mortality. However, it becomes obvious that regulation of telomere length is very complex and involves multiple processes. For example, the "end replication problem" during DNA replication as well as oxidative stress are responsible for the shortening of telomeres. In contrast, telomerase activity can potentially counteract telomere shortening when it is able to access and interact with telomeres. However, while highly active during development and in cancer cells, the enzyme is down-regulated in most human somatic cells with a few exceptions such as human lymphocytes. In addition, telomeres can be transcribed, and the transcription products called TERRA are involved in telomere length regulation.Thus, telomere length and their integrity are regulated at many different levels, and we only start to understand this process under conditions of increased oxidative stress, inflammation and during diseases as well as the ageing process.This chapter aims to describe our current state of knowledge on telomeres and telomerase and their regulation in order to better understand their role for the ageing process.

Telomere length and telomerase activity during folliculogenesis in mammals

Telomeres are repetitive non-coding DNA sequences located at the ends of chromosomes in eukaryotic cells. Their most important function is to protect chromosome ends from being recognized as DNA damage. They are also implicated in meiosis and synapse formation. The length of telomeres inevitably shortens at the end of each round of DNA replication and, also, as a consequence of the exposure to oxidative stress and/or genotoxic agents. The enzyme telomerase contributes to telomere lengthening. It has been reported that telomerase is exclusively expressed in germ cells, granulosa cells, early embryos, stem cells, and various types of cancerous cells. Granulosa cells undergo many mitotic divisions and either granulosa cells or oocytes are exposed to a variety of genotoxic agents throughout folliculogenesis; thus, telomerase plays an important role in the maintenance of telomere length. In this review article, we have comprehensively evaluated the studies focusing on the regulation of telomerase expression and activity, as well as telomere length, during folliculogenesis from primordial to antral follicles, in several mammalian species including mice, bovines, and humans. Also, the possible relationships between female infertility caused by follicular development defects and alterations in the telomeres and/or telomerase activity are discussed.

Telomere chromatin establishment and its maintenance during mammalian development

Telomeres are specialized structures that evolved to protect the end of linear chromosomes from the action of the cell DNA damage machinery. They are composed of tandem arrays of repeated DNA sequences with a specific heterochromatic organization. The length of telomeric repeats is dynamically regulated and can be affected by changes in the telomere chromatin structure. When telomeres are not properly controlled, the resulting chromosomal alterations can induce genomic instability and ultimately the development of human diseases, such as cancer. Therefore, proper establishment, regulation, and maintenance of the telomere chromatin structure are required for cell homeostasis. Here, we review the current knowledge on telomeric chromatin dynamics during cell division and early development in mammals, and how its proper regulation safeguards genome stability.

Transcription of highly repetitive tandemly organized DNA in amphibians and birds: A historical overview and modern concepts

Tandemly organized highly repetitive DNA sequences are crucial structural and functional elements of eukaryotic genomes. Despite extensive evidence, satellite DNA remains an enigmatic part of the eukaryotic genome, with biological role and significance of tandem repeat transcripts remaining rather obscure. Data on tandem repeats transcription in amphibian and avian model organisms is fragmentary despite their genomes being thoroughly characterized. Review systematically covers historical and modern data on transcription of amphibian and avian satellite DNA in somatic cells and during meiosis when chromosomes acquire special lampbrush form. We highlight how transcription of tandemly repetitive DNA sequences is organized in interphase nucleus and on lampbrush chromosomes. We offer LTR-activation hypotheses of widespread satellite DNA transcription initiation during oogenesis. Recent explanations are provided for the significance of high-yield production of non-coding RNA derived from tandemly organized highly repetitive DNA. In many cases the data on the transcription of satellite DNA can be extrapolated from lampbrush chromosomes to interphase chromosomes. Lampbrush chromosomes with applied novel technical approaches such as superresolution imaging, chromosome microdissection followed by high-throughput sequencing, dynamic observation in life-like conditions provide amazing opportunities for investigation mechanisms of the satellite DNA transcription.

Long noncoding RNAs in aging and age-related diseases

Aging is the universal, intrinsic, genetically-controlled, evolutionarily-conserved and time-dependent intricate biological process characterised by the cumulative decline in the physiological functions and their coordination in an organism after the attainment of adulthood resulting in the imbalance of neurological, immunological and metabolic functions of the body. Various biological processes and mechanisms along with altered levels of mRNAs and proteins have been reported to be involved in the progression of aging. It is one of the major risk factors in the patho-physiology of various diseases and disorders. Recently, the discovery of pervasive transcription of a vast pool of heterogeneous regulatory noncoding RNAs (ncRNAs), including small ncRNAs (sncRNAs) and long ncRNAs (lncRNAs), in the mammalian genome have provided an alternative way to study and explore the missing links in the aging process, its mechanism(s) and related diseases in a whole new dimension. The involvement of small noncoding RNAs in aging and age-related diseases have been extensively studied and recently reviewed. However, lncRNAs, whose function is far less explored in relation to aging, have emerged as a class of major regulators of genomic functions. Here, we have described some examples of known as well as novel lncRNAs that have been implicated in the progression of the aging process and age-related diseases. This may further stimulate research on noncoding RNAs and the aging process.

A single-cell assay for telomere DNA content shows increasing telomere length heterogeneity, as well as increasing mean telomere length in human spermatozoa with advancing age

PURPOSE

The effect of age on telomere length heterogeneity in men has not been studied previously. Our aims were to determine the relationship between variation in sperm telomere length (STL), men's age, and semen parameters in spermatozoa from men undergoing in vitro fertilization (IVF) treatment.

METHODS

To perform this prospective cross-sectional pilot study, telomere length was estimated in 200 individual spermatozoa from men undergoing IVF treatment at the NYU Fertility Center. A novel single-cell telomere content assay (SCT-pqPCR) measured telomere length in individual spermatozoa.

RESULTS

Telomere length among individual spermatozoa within an ejaculate varies markedly and increases with age. Older men not only have longer STL but also have more variable STL compared to younger men. STL from samples with normal semen parameters was significantly longer than that from samples with abnormal parameters, but STL did not differ between spermatozoa with normal versus abnormal morphology.

CONCLUSION

The marked increase in STL heterogeneity as men age is consistent with a role for ALT during spermatogenesis. No data have yet reported the effect of age on STL heterogeneity. Based on these results, future studies should expand this modest sample size to search for molecular evidence of ALT in human testes during spermatogenesis.

Role of TERRA in the regulation of telomere length

Telomere dysfunction is closely associated with human diseases such as cancer and ageing. Inappropriate changes in telomere length and/or structure result in telomere dysfunction. Telomeres have been considered to be transcriptionally silent, but it was recently demonstrated that mammalian telomeres are transcribed into telomeric repeat-containing RNA (TERRA). TERRA, a long non-coding RNA, participates in the regulation of telomere length, telomerase activity and heterochromatinization. The correct regulation of telomere length may be crucial to telomeric homeostasis and functions. Here, we summarize recent advances in our understanding of the crucial role of TERRA in the maintenance of telomere length, with focus on the variety of mechanisms by which TERRA is involved in the regulation of telomere length. This review aims to enable further understanding of how TERRA-targeted drugs can target telomere-related diseases.

On the origin of Robertsonian fusions in nature: evidence of telomere shortening in wild house mice

The role of telomere shortening to explain the occurrence of Robertsonian (Rb) fusions, as well as the importance of the average telomere length vs. the proportion of short telomeres, especially in nature populations, is largely unexplored. In this study, we have analysed telomere shortening in nine wild house mice from the Barcelona Rb system with diploid numbers ranging from 29 to 40 chromosomes. We also included two standard (2n=40) laboratory mice for comparison. Our data showed that the average telomere length (considering all chromosomal arms) is influenced by both the diploid number and the origin of the mice (wild vs. laboratory). In detail, we detected that wild mice from the Rb Barcelona system (fused and standard) present shorter telomeres than standard laboratory mice. However, only wild mice with Rb fusions showed a high proportion of short telomeres (only in p-arms), thus revealing the importance of telomere shortening in the origin of the Rb fusions in the Barcelona system. Overall, our study confirms that the number of critically short telomeres, and not a simple reduction in the average telomere length, is more likely to lead to the origin of Rb fusions in the Barcelona system and ultimately in nature.

Antiproliferative effect of the isoquinoline alkaloid papaverine in hepatocarcinoma HepG-2 cells--inhibition of telomerase and induction of senescence

Cancer cells are often immortal through up-regulation of the hTERT gene, which encodes the catalytic subunit of a special reverse transcriptase to overcome end-replication problem of chromosomes. This study demonstrates that papaverine, an isoquinoline alkaloid from the Papaveraceae, can overcome telomerase dependent immortality of HepG-2 cells that was used as a model of hepatocarcinoma. Although this alkaloid does not directly interact with telomeric sequences, papaverine inhibits telomerase through down-regulation of hTERT, which was analysed using thermal FRET and qRT-PCR, respectively. The IC50 values for the reduction of both telomerase activity and hTERT expression was 60 µM, while IC50 for cytotoxicity was 120 µM. Repeated treatments of the cells with very low non-toxic concentrations of papaverine resulted in growth arrest and strong reduction of population doublings after 40 days. This treatment induced senescent morphology in HepG-2 cells, which was evaluated by beta-galactosidase staining. Altogether, papaverine can be regarded as a promising model compound for drug design targeting cancer development.