Telomere length in male germ cells is inversely correlated with telomerase activity

Chromosome ends and the theory of marginotomy: implications for reproduction

Telomeres are the protective structures located at the ends of linear chromosomes. They were first described in the 1930s, but their biology remained unexplored until the early 70s, when Alexey M. Olovnikov, a theoretical biologist, suggested that telomeres cannot be fully copied during DNA replication. He proposed a theory that linked this phenomenon with the limit of cell proliferation capacity and the "duration of life" (theory of marginotomy), and suggested a potential of telomere lenghthening for the prevention of aging (anti-marginotomy). The impact of proliferative telomere shortening on life expectancy was later confirmed. In humans, telomere shortening is counteracted by telomerase, an enzyme that is undetectable in most adult somatic cells, but present in cancer cells and adult and embryonic stem and germ cells. Although telomere length dynamics are different in male and female gametes during gametogenesis, telomere lengths are reset at the blastocyst stage, setting the initial length of the species. The role of the telomere pathway in reproduction has been explored for years, mainly because of increased infertility resulting from delayed childbearing. Short telomere length in ovarian somatic cells is associated to decreased fertility and higher aneuploidy rates in embryos. Consequently, there is a growing interest in telomere lengthening strategies, aimed at improving fertility. It has also been observed that lifestyle factors can affect telomere length and improve fertility outcomes. In this review, we discuss the implications of telomere theory in fertility, especially in oocytes, spermatozoa, and embryos, as well as therapies to enhance reproductive success.

Telomeric function and regulation during male meiosis in mice and humans

BACKGROUND

Telomeres are unique structures situated at the ends of chromosomes. Preserving the structure and function of telomeres is essential for maintaining genomic stability and promoting genetic diversity during male meiosis in mammals.

MATERIAL-METHODS

This review compiled recent literature on the function and regulation of telomeres during male meiosis in both mice and humans, and also highlighted the critical roles of telomeres in reproductive biology and medicine.

RESULTS-DISCUSSION

Various structures, consisting of the LINC complex (SUN-KASH), SPDYA-CDK2, TTM trimer (TERB1-TERB2-MAJIN), and shelterin, are critical in controlling telomeric activities, such as nuclear envelope attachment and bouquet formation. Other than telomere-related proteins, cohesins and genes responsible for regulating telomere function are also highlighted, though the exact mechanism remains unclear. The gene-mutant mouse models with meiotic defects directly reveal the essential roles of telomeres in male meiosis. Recently reported mutant genes associated with telomere activity in clinical practice have also been illustrated in detail.

CONCLUSIONS

Proper regulation of telomere activities is essential for male meiosis progression in mice and humans.

Oxidative Stress, Telomere Length and Telomerase Activity in Spermatogenesis Disorders (Review of Scientific Activity)

The paper systematizes the available data on the study of oxidative stress, the relative length of telomeres, and telomerase activity in male infertility and disorders of spermatogenesis. The study of telomeres, the structures that protect chromosome ends and genome integrity, is of interest for researchers in various fields, from cell biology and epidemiology to ecology and evolutionary biology. The review includes our own data on the study of the relative length of telomeres, oxidative stress, and telomerase activity and reflects modern ideas about the importance of these structures both in the maintenance of genome stability during cell division and in gametogenesis and reproduction. Many studies indicate the role of oxidative stress in the pathogenesis of various diseases, including male infertility. In turn, studies of telomeres as a biomarker of male infertility are insufficient, and the results obtained are extremely controversial and require deeper knowledge about the mechanisms underlying the dynamics of telomere length.

Telomere Length, a New Biomarker of Male (in)Fertility? A Systematic Review of the Literature

Male factors are suspected in around half cases of infertility, of which up to 40% are diagnosed as idiopathic. In the context of a continuously increased resort to ART and increased decline of semen parameters, it is of greatest interest to evaluate an additional potential biomarker of sperm quality. According to PRISMA guidelines, this systematic review of the literature selected studies evaluating telomere length in sperm and/or in leukocytes as a potential male fertility biomarker. Twenty-two publications (3168 participants) were included in this review of experimental evidence. For each study, authors determined if there was a correlation between telomere length and semen parameters or fertility outcomes. Of the 13 studies concerning sperm telomere length (STL) and semen parameters, ten found an association between short STL and altered parameters. Concerning the impact of STL on ART results, the data are conflicting. However, eight of the 13 included studies about fertility found significantly longer sperm telomeres in fertile men than in infertile men. In leukocytes, the seven studies reported conflicting findings. Shorter sperm telomeres appear to be associated with altered semen parameters or male infertility. Telomere length may be considered as a new molecular marker of spermatogenesis and sperm quality, and thus is related to male fertility potential. However, additional studies are needed to define the place of the STL in the assessment of individual fertility.

Functional association between telomeres, oxidation and mitochondria

Prior research has substantiated the vital role of telomeres in human fertility. Telomeres are prerequisites for maintaining the integrity of chromosomes by preventing the loss of genetic material following replication events. Little is known about the association between sperm telomere length and mitochondrial capacity involving its structure and functions. Mitochondria are structurally and functionally distinct organelles that are located on the spermatozoon's midpiece. Mitochondria produce adenosine triphosphate (ATP) through oxidative phosphorylation (OXPHOS), which is necessary for sperm motility and generate reactive oxygen species (ROS). While a moderate concentration of ROS is critical for egg-sperm fusion, and fertilization, excessive ROS generation is primarily related to telomere shortening, sperm DNA fragmentation, and alterations in the methylation pattern leading to male infertility. This review aims to highlight the functional connection between mitochondria biogenesis and telomere length in male infertility, as mitochondrial lesions have a damaging impact on telomere length, leading both to telomere lengthening and reprogramming of mitochondrial biosynthesis. Furthermore, it aims to shed light on how both inositol and antioxidants can positively affect male fertility.

The Impact of Lifestyle on Sperm Function, Telomere Length, and IVF Outcomes

Many risk factors can potentially influence sperm quality. Telomeres confer stability on the chromosome and their dysfunction has been implicated in conditions such as cancer, aging, and lifestyle. The impact of lifestyle on sperm cell telomeres is unclear. The objectives of this study were to evaluate the impact of lifestyle behaviors on telomere length in sperm and to follow the correlation with pregnancy outcomes in patients undergoing in vitro fertilization (IVF). In this prospective observational study, sperm was analyzed for telomere length (TL). Men were asked to report lifestyle behaviors including occupation (physical or sedentary), smoking duration and amount, physical activity, dietary habits, and where they keep their cellular phone (bag, pants, or shirt pocket). Correlations among semen analysis, TL, men's habits, and embryo quality and pregnancy outcomes were evaluated. Among 34 patients recruited, 12 had longer TL and 13 shorter TL. Sperm motility was negatively correlated with TL (Pearson correlation = -.588, p = .002). Smoking adversely affected native sperm motility (53% motility in nonsmokers vs. 37% in smokers; p = .006). However, there was no significant impact on TL. The group with longer telomeres demonstrated significant association with healthy diet (10/12 vs. 6/13; p = .05) and a trend toward more sports activity, weekly (16/84 vs. 7/91; p = .04) compared with the shorter telomeres group. This study suggests that lifestyle, healthy diet, and sports activity are associated with long telomeres in sperm. Sperm quality is also influenced by patients' habits. The study strongly recommends maintaining a healthy lifestyle to preserve general health and fertility.

Post-transcriptional diversity in riboproteins and RNAs in aging and cancer

Post-transcriptional (PtscM) and post-translational (PtrnM) modifications of nucleotides and amino acids are covalent modifications able to change physio-chemical properties of RNAs and proteins. In the ribosome, the adequate assembly of rRNAs and ribosomal protein subunits in the nucleolus ensures suitable translational activity, with protein synthesis tuned according to intracellular demands of energy production, replication, proliferation, and growth. Disruption in the regulatory control of PtscM and PtrnM can impair ribosome biogenesis and ribosome function. Ribosomal impairment may, in turn, impact the synthesis of proteins engaged in functions as varied as telomere maintenance, apoptosis, and DNA repair, as well as intersect with mitochondria and telomerase activity. These cellular processes often malfunction in carcinogenesis and senescence. Here we discuss regulatory mechanisms of PtscMs and PtrnMs on ribosomal function. We also address chemical modification in rRNAs and their impacts on cellular metabolism, replication control, and senescence. Further, we highlight similarities and differences of PtscMs and PtrnMs in ribosomal intermediates during aging and carcinogenesis. Understanding these regulatory mechanisms may uncover critical steps for the development of more efficient oncologic and anti-aging therapies.

Lrrc34 Interacts with Oct4 and Has an Impact on Telomere Length in Mouse Embryonic Stem Cells

Telomere length maintenance in pluripotent stem cells (PSCs) is a main characteristic and a major premise for their undifferentiated long-term survival. However, little is known about the factors that control telomere length and elongation in these cells. Here, I describe Lrrc34 (leucine-rich repeat 34) as a novel telomere length regulating gene in murine embryonic stem cells. Downregulation of Lrrc34 results in significant reduction of telomerase activity and telomere length over time while also influencing the expression of known telomere length-associated genes. Generating induced PSCs (iPSCs) with Lrrc34 as a fifth factor in classical Yamanaka reprogramming increases the efficiency but did not have an impact on telomere length in the resulting iPSCs. Moreover, Lrrc34 was found to interact with Oct4, connecting the pluripotency network to telomere length regulation.

Analysis of leukocyte and sperm telomere length in oligozoospermic men

Telomere length is considered one of the most relevant biological markers of genomic stability since it protects DNA from impairment and also ensures chromosome alignment during DNA replication. The negative impact of telomere shortening on sperm quality has been suggested as an important indicator of male infertility. Therefore, we aimed to assess leucocyte and sperm telomere length (LTL&STL), as well as sperm parameters, DNA damage and protamine deficiency in men with oligozoospermia as compared to fertile men. Our results demonstrated a significant reduction in sperm parameters (concentration, motility, morphology), LTL & STL and a significant increase in sperm DNA damage and protamine deficiency in oligozoospermic men compared with fertile individuals. These outcomes revealed that low sperm concentration in men is possibly a sign of impaired meiotic and/or meiotic division during the spermatogenesis process. It is not only associated with proper chromatin packaging but also with telomere length as a key player in the process of mitosis and meiosis, assisting in chromosomal alignment, pairing, synapsis and crossing over during spermatogenesis.

Characterization of tree shrew telomeres and telomerase

The use of tree shrews as experimental animals for biomedical research is a new practice. Several recent studies suggest that tree shrews are suitable for studying cancers, including breast cancer, glioblastoma, lung cancer, and hepatocellular carcinoma. However, the telomeres and the telomerase of tree shrews have not been studied to date. Here, we characterize telomeres and telomerase in tree shrews. The telomere length of tree shrews is approximately 23 kb, which is longer than that of primates and shorter than that of mice, and it is extended in breast tumor tissues according to Southern blot and flow-fluorescence in situ hybridization (FISH) analyses. Tree shrew spleen, bone marrow, testis, ovary, and uterus show high telomerase activities, which are increased in breast tumor tissues by telomeric repeat amplification protocol assays. The telomere length becomes shorter, and telomerase activity decreases with age. The tree shrew TERT and TERC are more highly similar to primates than to rodents. These findings lay a solid foundation for using tree shrews to study aging and cancers.

Reproductive aging and telomeres: Are women and men equally affected?

Successful reproduction is very important for individuals and for society. Currently, the human health span and lifespan are the object of intense and productive investigation with great achievements, compared to the last century. However, reproduction span does not progress concomitantly with lifespan. Reproductive organs age, decreasing the levels of sexual hormones, which are protectors of health through their action on several organs of the body. Thus, this is the starting point of the organismal decay and infertility. This starting point is easily detected in women. In men, it goes under the surface, undetected, but it goes, nevertheless. Regarding fertility, aging alters the hormonal equilibrium, decreases the potential of reproductive organs, diminishes the quality of the gametes and worsen the reproductive outcomes. All these events happen at a different pace and affecting different organs in women and men. The question is what molecular pathways are involved in reproductive aging and if there is a possible halting or even reversion of the aging events. Answers to all these points will be explained in the present review.

Telomere Length and Male Fertility

Over the past decade, telomeres have attracted increasing attention due to the role they play in human fertility. However, conflicting results have been reported on the possible association between sperm telomere length (STL) and leukocyte telomere length (LTL) and the quality of the sperm parameters. The aim of this study was to run a comprehensive study to investigate the role of STL and LTL in male spermatogenesis and infertility. Moreover, the association between the sperm parameters and 11 candidate single nucleotide polymorphisms (SNPs), identified in the literature for their association with telomere length (TL), was investigated. We observed no associations between sperm parameters and STL nor LTL. For the individual SNPs, we observed five statistically significant associations with sperm parameters: considering a p < 0.05. Namely, ACYP2˗rs11125529 and decreased sperm motility (p = 0.03); PXK˗rs6772228 with a lower sperm count (p = 0.02); NAF1˗rs7675998 with increased probability of having abnormal acrosomes (p = 0.03) and abnormal flagellum (p = 0.04); ZNF208˗rs8105767 and reduction of sperms with normal heads (p = 0.009). This study suggests a moderate involvement of telomere length in male fertility; however, in our analyses four SNPs were weakly associated with sperm variables, suggesting the SNPs to be pleiotropic and involved in other regulatory mechanisms independent of telomere homeostasis, but involved in the spermatogenic process.

Telomere Maintenance Pathway Activity Analysis Enables Tissue- and Gene-Level Inferences

Telomere maintenance is one of the mechanisms ensuring indefinite divisions of cancer and stem cells. Good understanding of telomere maintenance mechanisms (TMM) is important for studying cancers and designing therapies. However, molecular factors triggering selective activation of either the telomerase dependent (TEL) or the alternative lengthening of telomeres (ALT) pathway are poorly understood. In addition, more accurate and easy-to-use methodologies are required for TMM phenotyping. In this study, we have performed literature based reconstruction of signaling pathways for the ALT and TEL TMMs. Gene expression data were used for computational assessment of TMM pathway activities and compared with experimental assays for TEL and ALT. Explicit consideration of pathway topology makes bioinformatics analysis more informative compared to computational methods based on simple summary measures of gene expression. Application to healthy human tissues showed high ALT and TEL pathway activities in testis, and identified genes and pathways that may trigger TMM activation. Our approach offers a novel option for systematic investigation of TMM activation patterns across cancers and healthy tissues for dissecting pathway-based molecular markers with diagnostic impact.

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.

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.

Associations of sperm telomere length with semen parameters, clinical outcomes and lifestyle factors in human normozoospermic samples

BACKGROUND

Many studies have demonstrated that lifestyle factors can affect sperm quality and fertility. Sperm telomere length (STL) has been reported as potential biomarker or sperm quality. However, no studies have investigated how lifestyle factors can affect STL and associated clinical outcomes.

OBJECTIVES

The purpose of this manuscript is to investigate any association between STL with lifestyle factors, semen parameters and clinical outcomes.

MATERIALS AND METHODS

Sperm telomere length was measured using real-time PCR in normozoospermic male partners (n = 66) of couples undergoing ART treatment. Each participant also completed a detailed questionnaire about general lifestyle. Linear regression univariate analysis and ANCOVA were performed to respectively determine correlations between STL and study parameters or identify statistically significant differences in STL while controlling for age, BMI and other factors.

RESULTS

Using a linear regression model, STL is positively correlated with in vitro fertilization success (n = 65, r = 0.37, P = .004) but not with embryo cleavage rates and post-implantation clinical outcomes including gestational age-adjusted birth weight. No associations were observed between STL and sperm count, concentration or progressive motility. We further found that STL did not associate age, BMI, health or lifestyle factors.

DISCUSSION

In somatic cells, the rate of telomere shortening is influenced by a number of lifestyle factors such as smoking, diet and occupation. However, little is known about how lifestyle factors affect STL and subsequently reproductive outcome. Out data suggest that STL might have an important role mechanistically for fertilization rate regardless of sperm parameters and lifestyle factors.

CONCLUSION

The results of this study demonstrate that STL is associated with in vitro fertilization rates, but not with semen parameters nor lifestyle factors. Further investigations are warranted to identify the potential variation of STL overtime to clarify its significance as a potential biomarker in ART.

DNA damage response manages cell cycle restriction of senile multipotent mesenchymal stromal cells

Multipotent mesenchymal stromal cells (MMSCs) are promising to treat a variety of traumatic and degenerative diseases. However, in vitro-passage aging induces cell cycle arrest and a series of genetic and biological changes, which greatly limits ex vivo cell number expansion and further clinical application of MMSCs. In most cases, DNA damage and DNA damage response (DDR) act as the main cause and executor of cellular senescence respectively. Mechanistically, DNA damage signals induce cell cycle arrest and DNA damage repair via DDR. If the DNA damage is indelible, MMSCs would entry into a permanent cell cycle arrest. It should be noted that apart from DDR signaling, certain proliferation or metabolism pathways are also occupied in DNA damage related cell cycle arrest. New findings of these aspects will also be summarized in this study. In summary, we aim to provide a comprehensive review of DDR associated cell cycle regulation and other major molecular signaling in the senescence of MMSCs. Above knowledge could contribute to improve the limited capacity of in vitro expansion of MMSCs, and then promote their clinical applications.

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.

Longitudinal analysis of somatic and germ-cell telomere dynamics in outbred mice

Although telomere length (TL) shortens with age in most tissues, an age-related increase in length has been described in sperm through a mechanism that is not yet fully understood. Changes in TL with age in the same individual have not been explored. This longitudinal study examines TL dynamics in somatic tissue and gametes during an entire lifespan in an outbred mouse population (from 8 to up to 114 weeks of age). Our findings indicate a reduced life expectancy in males compared to females (84.75 ± 9.23 vs. 113.16 ± 0.20 weeks) and significant variability in TL dynamics between individuals. While with aging, a clear reduction in TL was produced in somatic cells and oocytes, telomeres in sperm cells significantly lengthened. Finally, we found evidence indicating that telomere elongation in sperm during aging may be dependent on different mechanisms, such as the survival of spermatogonia with longer telomeres and the alternative lengthening of telomeres mechanism in meiotic and postmeiotic spermatogenic cells.

The paternal age at conception effect on offspring telomere length: mechanistic, comparative and adaptive perspectives

Telomeres are repeating DNA found at the ends of chromosomes that, in the absence of restorative processes, shorten with cell replications and are implicated as a cause of senescence. It appears that sperm telomere length (TL) increases with age in humans, and as a result offspring of older fathers inherit longer telomeres. We review possible mechanisms underlying this paternal age at conception (PAC) effect on TL, including sperm telomere extension due to telomerase activity, age-dependent changes in the spermatogonial stem cell population (possibly driven by 'selfish' spermatogonia) and non-causal confounding. In contrast to the lengthening of TL with PAC, higher maternal age at conception appears to predict shorter offspring TL in humans. We review evidence for heterogeneity across species in the PAC effect on TL, which could relate to differences in statistical power, sperm production rates or testicular telomerase activity. Finally, we review the hypothesis that the PAC effect on TL may allow a gradual multi-generational adaptive calibration of maintenance effort, and reproductive lifespan, to local demographic conditions: descendants of males who reproduced at a later age are likely to find themselves in an environment where increased maintenance effort, allowing later reproduction, represents a fitness improving resource allocation.This article is part of the theme issue 'Understanding diversity in telomere dynamics'.

The contribution of parent-to-offspring transmission of telomeres to the heritability of telomere length in humans

Leukocyte telomere length (LTL) is a heritable trait with two potential sources of heritability (h²): inherited variation in non-telomeric regions (e.g., SNPs that influence telomere maintenance) and variability in the lengths of telomeres in gametes that produce offspring zygotes (i.e., "direct" inheritance). Prior studies of LTL h² have not attempted to disentangle these two sources. Here, we use a novel approach for detecting the direct inheritance of telomeres by studying the association between identity-by-descent (IBD) sharing at chromosome ends and phenotypic similarity in LTL. We measured genome-wide SNPs and LTL for a sample of 5069 Bangladeshi adults with substantial relatedness. For each of the 6318 relative pairs identified, we used SNPs near the telomeres to estimate the number of chromosome ends shared IBD, a proxy for the number of telomeres shared IBD (T_shared). We then estimated the association between T_shared and the squared pairwise difference in LTL ((ΔLTL)²) within various classes of relatives (siblings, avuncular, cousins, and distant), adjusting for overall genetic relatedness (ϕ). The association between T_shared and (ΔLTL)² was inverse among all relative pair types. In a meta-analysis including all relative pairs (ϕ > 0.05), the association between T_shared and (ΔLTL)² (P = 0.01) was stronger than the association between ϕ and (ΔLTL)² (P = 0.43). Our results provide strong evidence that telomere length (TL) in parental germ cells impacts TL in offspring cells and contributes to LTL h² despite telomere "reprogramming" during embryonic development. Applying our method to larger studies will enable robust estimation of LTL h² attributable to direct transmission of telomeres.

The protective function of noncoding DNA in genome defense of eukaryotic male germ cells

Peripheral and abundant noncoding DNA has been hypothesized to protect the genome and the central protein-coding sequences against DNA damage in somatic genome. In the cytosol, invading exogenous nucleic acids may first be deactivated by small RNAs encoded by noncoding DNA via mechanisms similar to the prokaryotic CRISPR-Cas system. In the nucleus, the radicals generated by radiation in the cytosol, radiation energy and invading exogenous nucleic acids are absorbed, blocked and/or reduced by peripheral heterochromatin, and damaged DNA in heterochromatin is removed and excluded from the nucleus to the cytoplasm through nuclear pore complexes. To further strengthen the hypothesis, this review summarizes the experimental evidence supporting the protective function of noncoding DNA in the genome of male germ cells. Based on these data, this review provides evidence supporting the protective role of noncoding DNA in the genome defense of sperm genome through similar mechanisms to those of the somatic genome.

Telomeres as a molecular marker of male infertility

In recent years, male infertility has become a growing social problem. Standard diagnostic procedures, based on assessing seminological parameters, are often insufficient to explain the causes of male infertility. Because of this, new markers with better clinical application are being sought. One of the promising markers seems to be an assessment of telomere length of sperm. Sperm telomeres, in contrast to somatic cells, are elongated as men age. The results of some studies suggest that telomere length may be relevant in the case of fertilization and normal embryo development. Literature reports indicate that there is a correlation between telomere length of sperm and abnormal sperm parameters. The measurement of telomere length using the method of quantitative PCR could become a new marker of spermatogenesis, which can be useful for evaluating male reproductive age.

The fetal programming of telomere biology hypothesis: an update

Research on mechanisms underlying fetal programming of health and disease risk has focused primarily on processes that are specific to cell types, organs or phenotypes of interest. However, the observation that developmental conditions concomitantly influence a diverse set of phenotypes, the majority of which are implicated in age-related disorders, raises the possibility that such developmental conditions may additionally exert effects via a common underlying mechanism that involves cellular/molecular ageing-related processes. In this context, we submit that telomere biology represents a process of particular interest in humans because, firstly, this system represents among the most salient antecedent cellular phenotypes for common age-related disorders; secondly, its initial (newborn) setting appears to be particularly important for its long-term effects; and thirdly, its initial setting appears to be plastic and under developmental regulation. We propose that the effects of suboptimal intrauterine conditions on the initial setting of telomere length and telomerase expression/activity capacity may be mediated by the programming actions of stress-related maternal-placental-fetal oxidative, immune, endocrine and metabolic pathways in a manner that may ultimately accelerate cellular dysfunction, ageing and disease susceptibility over the lifespan. This perspectives paper provides an overview of each of the elements underlying this hypothesis, with an emphasis on recent developments, findings and future directions.This article is part of the theme issue 'Understanding diversity in telomere dynamics'.

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.

Sperm telomere length in motile sperm selection techniques: A qFISH approach

Several studies have associated telomere shortening with alterations in reproductive function. The objective of the present study was to determine telomere length (TL) in spermatozoa selected by either density-gradient centrifugation (DGC) or swim-up. The analysis of TL was performed using quantitative fluorescent in situ hybridisation (qFISH) using PNA probes in combination with a chromatin decompaction protocol in sperm cells. Results of TL were 24.64 ± 5.00 Kb and 24.95 ± 4.60 Kb before and after DGC, respectively, and 19.59 ± 8.02 Kb and 20.22 ± 5.18 Kb before and after swim-up respectively. Sperm selected by DGC or swim-up did not show any significant differences in TL as compared to nonselected sperm (p > .05). Negative correlations between TL and sperm motility (r = -.308; p = .049) and concentration (r = -.353; p = .028) were found. Furthermore, exposure of sperm to increasing concentrations of hydrogen peroxide during incubation resulted in a reduction in TL. These data indicate that oxidative stress may be one of the main factors involved in the reduction of TL in sperm. Preliminary clinical results from patients included in this study indicate that TL was shorter in spermatozoa from couples who never achieved a pregnancy compared to couples who did achieve at least one natural pregnancy (p < .05); however, the clinical utility of this biomarker still needs to be confirmed in further studies.

Paternal and grandpaternal ages at conception and descendant telomere lengths in chimpanzees and humans

Telomeres are repeating DNA at chromosome ends. Telomere length (TL) declines with age in most human tissues, and shorter TL is thought to accelerate senescence. In contrast, older men have sperm with longer TL; correspondingly, older paternal age at conception (PAC) predicts longer TL in offspring. This PAC-effect could be a unique form of transgenerational genetic plasticity that modifies somatic maintenance in response to cues of recent ancestral experience. The PAC-effect has not been examined in any non-human mammals.

OBJECTIVES

Here, we examine the PAC-effect in chimpanzees (Pan troglodytes). The PAC-effect on TL is thought to be driven by continual production of sperm-the same process that drives increased de novo mutations with PAC. As chimpanzees have both greater sperm production and greater sperm mutation rates with PAC than humans, we predict that the PAC-effect on TL will be more pronounced in chimpanzees. Additionally we examine whether PAC predicts TL of grandchildren.

MATERIALS AND METHODS

TL were measured using qPCR from DNA from blood samples from 40 captive chimpanzees and 144 humans.

RESULTS

Analyses showed increasing TL with PAC in chimpanzees (p = .009) with a slope six times that in humans (p = .026). No associations between TL and grandpaternal ages were found in humans or chimpanzees-although statistical power was low.

DISCUSSION

These results suggest that sperm production rates across species may be a determinant of the PAC-effect on offspring TL. This raises the possibility that sperm production rates within species may influence the TL passed on to offspring.

Molecular Signatures of Primary Human Spermatogonial Progenitors and Its Neighboring Peritubular Stromal Compartment

In-depth understanding of human spermatogenesis requires studying specific molecular signatures and interactions of spermatogonia with other testicular cell populations, for which isolation of pure populations of different cell types is crucial. Here, we describe a technique to simultaneously enrich pure, multiple testicular cell populations, including spermatogonia, endothelial (TECs), and perivascular mesenchymal stem/stromal cells (TMSCs), from testicular tissue by flow cytometry using a combination of defined markers. Immunohistochemical studies, multicolor staining, and cell sorting followed by multiplex quantitative real-time polymerase chain reaction (qRT-PCR) analysis revealed that spermatogonia were highly enriched in the CD49f⁺CD49a^-HLA-ABC^-SSEA-4⁺ fraction of primary testicular cells. In contrast to spermatogonia, TMSCs and TECs were highly enriched in the CD49f⁺CD49a⁺HLA-ABC⁺CD144^- and CD49f⁺CD49a⁺HLA-ABC⁺CD144⁺subsets, respectively. The delineation was confirmed by the expression of specific stromal and endothelial key markers as well as by the differentiation and angiogenic capacity of the sorted populations. In this article, for the first time, we performed transcriptome profiling of highly enriched, freshly isolated human spermatogonia and compared their expression profile with that of TMSCs. Our RNA sequencing data favor the hypothesis that TMSCs are candidate niche components for spermatogonia. The composite genotype and phenotype of defined testicular cell populations combined with a robust isolation procedure from small biopsies contributes to a better understanding of cellular interactions and for the establishment of efficient culture techniques to maintain spermatogonial progenitors.

Effect of paternal overweight or obesity on IVF treatment outcomes and the possible mechanisms involved

Leukocyte telomere lengths (LTLs) are shorter in obese compared with normal weight people. However, it is not known whether sperm telomere length (STL) is related to obesity. The aim of the study was to evaluate the impact of men’s body mass index (BMI) on STL, embryo quality, and clinical outcomes in couples undergoing IVF. In total, 651 couples were recruited, including 345 men with a normal BMI and 306 men with an overweight BMI (normal BMI group: 20–25 kg/m²; overweight BMI group: >28 kg/m²). We found that couples with male’s BMI over 28 kg/m² exhibited a significantly lower fertilization rate, good-quality embryo rate and clinical pregnancy rate compared to their normal BMI counterparts. The mean STL in the overweight BMI group was also significantly shorter than that of the normal BMI group. The results also showed that individuals with higher BMI had higher ROS (Reactive oxygen species) content and sperm DNA fragmentation rate when compared with normal BMI individuals. Mitochondrial activity was also lower in the overweight BMI group than in the normal BMI group. This is the first report to find that STL is shorter in overweight/obese men, which may account for their poorer treatment outcomes in IVF cycles.

Hypoxia Inducible Factor 1 Alpha Is Expressed in Germ Cells throughout the Murine Life Cycle

Pluripotent stem cells of the early embryo, and germ line cells, are essential to ensure uncompromised development to adulthood as well as species propagation, respectively. Recently, the transcription factor hypoxia inducible factor 1 alpha (Hif1α) has been shown to have important roles in embryonic stem cells; in particular, regulation of conversion to glycolytic metabolism and, as we have shown, maintenance of functional levels of telomerase. In the present study, we sought to assess whether Hif1α was also expressed in the primitive cells of the murine embryo. We observed expression of Hif1α in pre-implantation embryos, specifically the 2-cell stage, morula, and blastocyst. Robust Hif1α expression was also observed in male and female primordial germ cells. We subsequently assessed whether Hif1α was expressed in adult male and female germ cells. In the testis, Hif1α was robustly expressed in spermatogonial cells, in both juvenile (6-week old) and adult (3-month old) males. In the ovaries, Hif1α was expressed in mature oocytes from adult females, as assessed both in situ and in individual oocytes flushed from super-ovulated females. Analysis of Hif1α transcript levels indicates a mechanism of regulation during early development that involves stockpiling of Hif1α protein in mature oocytes, presumably to provide protection from hypoxic stress until the gene is re-activated at the blastocyst stage. Together, these observations show that Hif1α is expressed throughout the life-cycle, including both the male and female germ line, and point to an important role for Hif1α in early progenitor cells.

Cold-inducible RNA-binding protein CIRP/hnRNP A18 regulates telomerase activity in a temperature-dependent manner

The telomerase is responsible for adding telomeric repeats to chromosomal ends and consists of the reverse transcriptase TERT and the RNA subunit TERC. The expression and activity of the telomerase are tightly regulated, and aberrant activation of the telomerase has been observed in >85% of human cancers. To better understand telomerase regulation, we performed immunoprecipitations coupled with mass spectrometry (IP-MS) and identified cold inducible RNA-binding protein (CIRP or hnRNP A18) as a telomerase-interacting factor. We have found that CIRP is necessary to maintain telomerase activities at both 32°C and 37°C. Furthermore, inhibition of CIRP by CRISPR-Cas9 or siRNA knockdown led to reduced telomerase activities and shortened telomere length, suggesting an important role of CIRP in telomere maintenance. We also provide evidence here that CIRP associates with the active telomerase complex through direct binding of TERC and regulates Cajal body localization of the telomerase. In addition, CIRP regulates the level of TERT mRNAs. At the lower temperature, TERT mRNA is upregulated in a CIRP-dependent manner to compensate for reduced telomerase activities. Taken together, these findings highlight the dual roles that CIRP plays in regulating TERT and TERC, and reveal a new class of telomerase modulators in response to hypothermia conditions.

G-quadruplexes as potential therapeutic targets for embryonal tumors

Embryonal tumors include a heterogeneous group of highly malignant neoplasms that primarily affect infants and children and are characterized by a high rate of mortality and treatment-related morbidity, hence improved therapies are clearly needed. G-quadruplexes are special secondary structures adopted in guanine (G)-rich DNA sequences that are often present in biologically important regions, e.g. at the end of telomeres and in the regulatory regions of oncogenes such as MYC. Owing to the significant roles that both telomeres and MYC play in cancer cell biology, G-quadruplexes have been viewed as emerging therapeutic targets in oncology and as tools for novel anticancer drug design. Several compounds that target these structures have shown promising anticancer activity in tumor xenograft models and some of them have entered Phase II clinical trials. In this review we examine approaches to DNA targeted cancer therapy, summarize the recent developments of G-quadruplex ligands as anticancer drugs and speculate on the future direction of such structures as a potential novel therapeutic strategy for embryonal tumors of the nervous system.

Meta-analysis of telomere length in 19,713 subjects reveals high heritability, stronger maternal inheritance and a paternal age effect

Telomere length (TL) has been associated with aging and mortality, but individual differences are also influenced by genetic factors, with previous studies reporting heritability estimates ranging from 34 to 82%. Here we investigate the heritability, mode of inheritance and the influence of parental age at birth on TL in six large, independent cohort studies with a total of 19,713 participants. The meta-analysis estimate of TL heritability was 0.70 (95% CI 0.64-0.76) and is based on a pattern of results that is highly similar for twins and other family members. We observed a stronger mother-offspring (r=0.42; P-value=3.60 × 10(-61)) than father-offspring correlation (r=0.33; P-value=7.01 × 10(-5)), and a significant positive association with paternal age at offspring birth (β=0.005; P-value=7.01 × 10(-5)). Interestingly, a significant and quite substantial correlation in TL between spouses (r=0.25; P-value=2.82 × 10(-30)) was seen, which appeared stronger in older spouse pairs (mean age ≥55 years; r=0.31; P-value=4.27 × 10(-23)) than in younger pairs (mean age<55 years; r=0.20; P-value=3.24 × 10(-10)). In summary, we find a high and very consistent heritability estimate for TL, evidence for a maternal inheritance component and a positive association with paternal age.

Analysis of sperm telomere length in men with idiopathic infertility

PURPOSE

Telomeres are multifunctional nucleoprotein domains with hexanucleotide tandem repeat (5' TTAGGG 3') sequences, which cap the chromosome ends. However, the role of telomere and its length in sperm with regard to fertility remains unknown.

METHODS

In this pilot study, we analyzed 32 idiopathic infertile men and 25 controls for sperm telomere length by quantitative polymerase chain reaction (Q-PCR), and correlated it with sperm DNA fragmentation index (DFI) and reactive oxygen species (ROS) levels.

RESULTS

The relative sperm mean telomere length (T/S) of infertile men was found to be significantly lower (p < 0.005) when compared to controls (0.674 ± 0.028 vs. 0.699 ± 0.030). None of the sperm parameters such as sperm count, forward motility, morphology, ROS, and DFI were found to correlate with the sperm telomere length.

CONCLUSION

Shorter telomeres in sperm may be one of the causative factors responsible for male infertility, but further detailed studies are needed to confirm these findings.

Prenatal stress, telomere biology, and fetal programming of health and disease risk

A substantial body of epidemiological, clinical, cellular, and molecular evidence converges to suggest that conditions during the intrauterine period of life play a critical role in developmental programming to influence subsequent health and susceptibility for common, complex disorders. Elucidation of the biological mechanisms underlying these effects is an area of considerable interest and investigation, and it is important to determine whether these mechanisms are distinct for different health outcomes or whether there are some common underlying pathways that may account for the effects of disparate prenatal and early postnatal conditions on various health and disease risk phenotypes. We propose that telomere biology may represent a common underlying mechanism connecting fetal programming and subsequent health outcomes. It appears that the initial establishment of telomere length and regulation of telomere homeostasis may be plastic and receptive to the influence of intrauterine and other early life conditions. Moreover, telomere homeostasis in various cell types may serve as a fundamental integrator and regulator of processes underlying cell genomic integrity and function, aging, and senescence over the life span. We advance the hypothesis that context- and time-inappropriate exposures to various forms of physiological stress (maternal-placental-fetal endocrine aberrations and immune, inflammatory, and oxidative stresses) during the intrauterine period of development may alter or program the telomere biology system in a manner that accelerates cellular dysfunction, aging, and disease susceptibility over the life span.

Characterization of telomeric repeats in metaphase chromosomes and interphase nuclei of Syrian Hamster Fibroblasts

Background

Rodents have been reported to contain large arrays of interstitial telomeric sequences (TTAGGG)n (ITS) located in pericentromeric heterochromatin. The relative sizes of telomeric sequences at the ends of chromosomes (TS) and ITS in Syrian hamster (Mesocricetus auratus) cells have not been evaluated yet, as well as their structural organization in interphase nuclei.

Results

FISH signal distribution analysis was performed on DAPI-banded metaphase chromosomes of Syrian hamster fibroblasts, and relative lengths of telomere signals were estimated. Besides well-distinguished FISH signals from ITS located on chromosomes ##2, 4, 14, 20 and X that we reported earlier, low-intensity FISH signals were visualized with different frequency of detection on all other metacentric chromosomes excluding chromosome #21. The analysis of 3D-distribution of TS in interphase nuclei demonstrated that some TS foci formed clearly distinguished associations (2–3 foci in a cluster) in the nuclei of cells subjected to FISH or transfected with the plasmid expressing telomeric protein TRF1 fused with GFP. In G0 and G1/early S-phase, the average total number of GFP-TRF1 foci per nucleus was less than that of PNA FISH foci in the corresponding cell cycle phases suggesting that TRF1 overexpression might contribute to the fusion of neighboring telomeres. The mean total number of GFP-TRF1 and FISH foci per nucleus was increased during the transition from G0 to G1/early S-phase that might be the consequence of duplication of some TS.

Conclusions

The relative lengths of TS in Syrian hamster cells were found to be moderately variable. All but one metacentric chromosomes contain ITS in pericentromeric heterochromatin indicating that significant rearrangements of ancestral genome occurred in evolution. Visualization of GFP-TRF1 fibrils that formed bridges between distinct telomeric foci allowed suggesting that telomere associations observed in interphase cells are reversible. The data obtained in the study provide the further insight in the structure and dynamics of telomeric sequences in somatic mammalian cells.

Cell division and aging of the organism

The capacity to regenerate cell compartments through cell proliferation is an important characteristic of many developed metazoan tissues. Pre- and post-natal development proceeds through the modifications occurring during cell division. Experiments with cultivated cells showed that cell proliferation originates changes in cell functions and coordinations that contribute to aging and senescence. The implications of the finite cell proliferation to aging of the organism is not the accumulation of cells at the end of their life cycle, but rather the drift in cell function created by cell division. Comparative gerontology shows that the regulation of the length of telomeres has no implications for aging. On the other hand there are interspecies differences in regard to the somatic cell division potential that seem to be related with the "plasticity" of the genome and with longevity, which should be viewed independently of the aging phenomenon. Telomeres may play a role in this plasticity through the regulation of chromosome recombination, and via the latter also in development.