Telomeres in mammalian male germline cells

Male reproductive ageing: a radical road to ruin

In modern post-transition societies, we are reproducing later and living longer. While the impact of age on female reproductive function has been well studied, much less is known about the intersection of age and male reproduction. Our current understanding is that advancing age brings forth a progressive decline in male fertility accompanied by a reduction in circulating testosterone levels and the appearance of age-dependent reproductive pathologies including benign prostatic hypertrophy and erectile dysfunction. Paternal ageing is also associated with a profound increase in sperm DNA damage, the appearance of multiple epigenetic changes in the germ line and an elevated mutational load in the offspring. The net result of such changes is an increase in the disease burden carried by the progeny of ageing males, including dominant genetic diseases such as Apert syndrome and achondroplasia, as well as neuropsychiatric conditions including autism and spontaneous schizophrenia. The genetic basis of these age-related effects appears to involve two fundamental mechanisms. The first is a positive selection mechanism whereby stem cells containing mutations in a mitogen-activated protein kinase pathway gain a selective advantage over their non-mutant counterparts and exhibit significant clonal expansion with the passage of time. The second is dependent on an age-dependent increase in oxidative stress which impairs the steroidogenic capacity of the Leydig cells, disrupts the ability of Sertoli cells to support the normal differentiation of germ cells, and disrupts the functional and genetic integrity of spermatozoa. Given the central importance of oxidative stress in defining the impact of chronological age on male reproduction, there may be a role for antioxidants in the clinical management of this process. While animal studies are supportive of this strategy, carefully designed clinical trials are now needed if we are to realize the therapeutic potential of this approach in a clinical context.

Impact of Advanced Paternal Age on Fertility and Risks of Genetic Disorders in Offspring

The average age of fathers at first pregnancy has risen significantly over the last decade owing to various variables, including a longer life expectancy, more access to contraception, later marriage, and other factors. As has been proven in several studies, women over 35 years of age have an increased risk of infertility, pregnancy problems, spontaneous abortion, congenital malformations, and postnatal issues. There are varying opinions on whether a father's age affects the quality of his sperm or his ability to father a child. First, there is no single accepted definition of old age in a father. Second, much research has reported contradictory findings in the literature, particularly concerning the most frequently examined criteria. Increasing evidence suggests that the father's age contributes to his offspring's higher vulnerability to inheritable diseases. Our comprehensive literature evaluation shows a direct correlation between paternal age and decreased sperm quality and testicular function. Genetic abnormalities, such as DNA mutations and chromosomal aneuploidies, and epigenetic modifications, such as the silencing of essential genes, have all been linked to the father's advancing years. Paternal age has been shown to affect reproductive and fertility outcomes, such as the success rate of in vitro fertilisation (IVF), intracytoplasmic sperm injection (ICSI), and premature birth rate. Several diseases, including autism, schizophrenia, bipolar disorders, and paediatric leukaemia, have been linked to the father's advanced years. Therefore, informing infertile couples of the alarming correlations between older fathers and a rise in their offspring's diseases is crucial, so that they can be effectively guided through their reproductive years.

Transgenerational inheritance: how impacts to the epigenetic and genetic information of parents affect offspring health

BACKGROUND

A defining feature of sexual reproduction is the transmission of genomic information from both parents to the offspring. There is now compelling evidence that the inheritance of such genetic information is accompanied by additional epigenetic marks, or stable heritable information that is not accounted for by variations in DNA sequence. The reversible nature of epigenetic marks coupled with multiple rounds of epigenetic reprogramming that erase the majority of existing patterns have made the investigation of this phenomenon challenging. However, continual advances in molecular methods are allowing closer examination of the dynamic alterations to histone composition and DNA methylation patterns that accompany development and, in particular, how these modifications can occur in an individual’s germline and be transmitted to the following generation. While the underlying mechanisms that permit this form of transgenerational inheritance remain unclear, it is increasingly apparent that a combination of genetic and epigenetic modifications plays major roles in determining the phenotypes of individuals and their offspring.

OBJECTIVE AND RATIONALE

Information pertaining to transgenerational inheritance was systematically reviewed focusing primarily on mammalian cells to the exclusion of inheritance in plants, due to inherent differences in the means by which information is transmitted between generations. The effects of environmental factors and biological processes on both epigenetic and genetic information were reviewed to determine their contribution to modulating inheritable phenotypes.

SEARCH METHODS

Articles indexed in PubMed were searched using keywords related to transgenerational inheritance, epigenetic modifications, paternal and maternal inheritable traits and environmental and biological factors influencing transgenerational modifications. We sought to clarify the role of epigenetic reprogramming events during the life cycle of mammals and provide a comprehensive review of how the genomic and epigenomic make-up of progenitors may determine the phenotype of its descendants.

OUTCOMES

We found strong evidence supporting the role of DNA methylation patterns, histone modifications and even non-protein-coding RNA in altering the epigenetic composition of individuals and producing stable epigenetic effects that were transmitted from parents to offspring, in both humans and rodent species. Multiple genomic domains and several histone modification sites were found to resist demethylation and endure genome-wide reprogramming events. Epigenetic modifications integrated into the genome of individuals were shown to modulate gene expression and activity at enhancer and promoter domains, while genetic mutations were shown to alter sequence availability for methylation and histone binding. Fundamentally, alterations to the nuclear composition of the germline in response to environmental factors, ageing, diet and toxicant exposure have the potential to become hereditably transmitted.

WIDER IMPLICATIONS

The environment influences the health and well-being of progeny by working through the germline to introduce spontaneous genetic mutations as well as a variety of epigenetic changes, including alterations in DNA methylation status and the post-translational modification of histones. In evolutionary terms, these changes create the phenotypic diversity that fuels the fires of natural selection. However, rather than being adaptive, such variation may also generate a plethora of pathological disease states ranging from dominant genetic disorders to neurological conditions, including spontaneous schizophrenia and autism.

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.

Should we consider telomere length and telomerase activity in male factor infertility?

PURPOSE OF REVIEW

The purpose of this review is to analyze what is known to date about the relation between telomeres and male fertility, and if it is possible for telomeres, or elements related to them, to be used as new prognostic biomarkers in fertility treatment.

RECENT FINDINGS

Cells in germ series, including spermatozoids, have longer telomeres (10-20 kb), and do not seem to undergo the shortening that takes place in somatic cells with age as they present telomerase activity. Longer telomere length found in the sperm of older fathers, influences their offspring possessing cells with longer telomere length. Infertile patients have spermatozoids with shorter telomere length than fertile people, but telomere length does neither correlate with the sperm concentration, mobility or morphology, nor with the DNA fragmentation indices (DFI) of spermatozoids. Embryo quality rate and transplantable embryo rate are related with the telomere length of spermatozoids (STL), but pregnancy rates are not affected.

SUMMARY

Telomere length and telomerase levels can be used as biomarkers of male fertility. Higher STL can have beneficial effects on fertility, thus the use of spermatozoids with longer telomere length in an assisted reproduction technique (ART) could be one way of solving some infertility cases.

Preferable location of chromosomes 1, 29, and X in bovine spermatozoa

Chromosome positioning in sperm nucleus may have a functional significance by influencing the sequence of post-fertilization events. In this study we present data on preferential locations of chromosomes 1, 29 and X in Bos taurus spermatozoa. Here we demonstrate that the position of X chromosome in the sperm nucleus is more restricted as compared to the position of chromosome 1, which is about of the same size. Our data support the concept of the functional significance of genome architecture in male germline cells.

Sub-fertile sperm cells exemplify telomere dysfunction

PURPOSE

The purpose of this study was to evaluate telomere homeostasis in sub-fertile compared to fertile human sperm.

METHODS

This observational, comparative study included 16 sub-fertile men who required intracytoplasmic sperm injection and 10 fertile men. At least 100 sperm cells from each participant were assessed. Main outcome measures were telomere length and telomere aggregates. Telomerase RNA component (TERC) copy number and telomere capture were assessed using fluorescence in situ hybridization technique and human telomerase reverse transcriptase (hTERT) using immunohistochemistry.

RESULTS

Clinical backgrounds were similar. The percentage of sperm cells with shorter telomeres was higher among the sub-fertile compared to the fertile participants (3.3 ± 3.1 vs. 0.6 ± 1.2%, respectively; P < 0.005). The percentage of cells with telomere aggregates was significantly higher in the sub-fertile group (15.12 ± 3.73 vs. 4.73 ± 3.73%; P < 0.005). TERC gene copy number was similar between groups. The percentage of cells that were positive for hTERT was lower in the sub-fertile group (3.81 ± 1.27 vs. 8.42 ± 1.80%; P < 0.005). Telomere capture rates were higher among the sub-fertile sperm cells (P < 0.005).

CONCLUSIONS

Sub-fertile sperm cells have short telomeres that are elongated by the alternative pathway of telomere capture. Dysfunctional telomeres may affect sperm fertilizability.

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.

Altered Crossover Distribution and Frequency in Spermatocytes of Infertile Men with Azoospermia

During meiosis, homologous chromosomes pair to facilitate the exchange of DNA at crossover sites along the chromosomes. The frequency and distribution of crossover formation are tightly regulated to ensure the proper progression of meiosis. Using immunofluorescence techniques, our group and others have studied the meiotic proteins in spermatocytes of infertile men, showing that this population displays a reduced frequency of crossovers compared to fertile men. An insufficient number of crossovers is thought to promote chromosome missegregation, in which case the faulty cell may face meiotic arrest or contribute to the production of aneuploid sperm. Increasing evidence in model organisms has suggested that the distribution of crossovers may also be important for proper chromosome segregation. In normal males, crossovers are shown to be rare near centromeres and telomeres, while frequent in subtelomeric regions. Our study aims to characterize the crossover distribution in infertile men with non-obstructive (NOA) and obstructive azoospermia (OA) along chromosomes 13, 18 and 21. Eight of the 16 NOA men and five of the 21 OA men in our study displayed reduced crossover frequency compared to control fertile men. Seven NOA men and nine OA men showed altered crossover distributions on at least one of the chromosome arms studied compared to controls. We found that although both NOA and OA men displayed altered crossover distributions, NOA men may be at a higher risk of suffering both altered crossover frequencies and distributions compared to OA men. Our data also suggests that infertile men display an increase in crossover formation in regions where they are normally inhibited, specifically near centromeres and telomeres. Finally, we demonstrated a decrease in crossovers near subtelomeres, as well as increased average crossover distance to telomeres in infertile men. As telomere-guided mechanisms are speculated to play a role in crossover formation in subtelomeres, future studies linking crossover distribution with telomere integrity and sperm aneuploidy may provide new insight into the mechanisms underlying male infertility.

High telomerase is a hallmark of undifferentiated spermatogonia and is required for maintenance of male germline stem cells

Telomerase inactivation causes loss of the male germline in worms, fish, and mice, indicating a conserved dependence on telomere maintenance in this cell lineage. Here, using telomerase reverse transcriptase (Tert) reporter mice, we found that very high telomerase expression is a hallmark of undifferentiated spermatogonia, the mitotic population where germline stem cells reside. We exploited these high telomerase levels as a basis for purifying undifferentiated spermatogonia using fluorescence-activated cell sorting. Telomerase levels in undifferentiated spermatogonia and embryonic stem cells are comparable and much greater than in somatic progenitor compartments. Within the germline, we uncovered an unanticipated gradient of telomerase activity that also enables isolation of more mature populations. Transcriptomic comparisons of Tert(High) undifferentiated spermatogonia and Tert(Low) differentiated spermatogonia by RNA sequencing reveals marked differences in cell cycle and key molecular features of each compartment. Transplantation studies show that germline stem cell activity is confined to the Tert(High) cKit(-) population. Telomere shortening in telomerase knockout strains causes depletion of undifferentiated spermatogonia and eventual loss of all germ cells after undifferentiated spermatogonia drop below a critical threshold. These data reveal that high telomerase expression is a fundamental characteristic of germline stem cells, thus explaining the broad dependence on telomerase for germline immortality in metazoans.

Maternal and genetic factors determine early life telomere length

In a broad range of species--including humans--it has been demonstrated that telomere length declines throughout life and that it may be involved in cell and organismal senescence. This potential link to ageing and thus to fitness has triggered recent interest in understanding how variation in telomere length is inherited and maintained. However, previous studies suffer from two main drawbacks that limit the possibility of understanding the relative importance of genetic, parental and environmental influences on telomere length variation. These studies have been based on (i) telomere lengths measured at different time points in different individuals, despite the fact that telomere length changes over life, and (ii) parent-offspring regression techniques, which do not enable differentiation between genetic and parental components of inheritance. To overcome these drawbacks, in our study of a songbird, the great reed warbler, we have analysed telomere length measured early in life in both parents and offspring and applied statistical models (so-called 'animal models') that are based on long-term pedigree data. Our results showed a significant heritability of telomere length on the maternal but not on the paternal side, and that the mother's age was positively correlated with their offspring's telomere length. Furthermore, the pedigree-based analyses revealed a significant heritability and an equally large maternal effect. Our study demonstrates strong maternal influence on telomere length and future studies now need to elucidate possible underlying factors, including which types of maternal effects are involved.

Effects of increased paternal age on sperm quality, reproductive outcome and associated epigenetic risks to offspring

Over the last decade, there has been a significant increase in average paternal age when the first child is conceived, either due to increased life expectancy, widespread use of contraception, late marriages and other factors. While the effect of maternal ageing on fertilization and reproduction is well known and several studies have shown that women over 35 years have a higher risk of infertility, pregnancy complications, spontaneous abortion, congenital anomalies, and perinatal complications. The effect of paternal age on semen quality and reproductive function is controversial for several reasons. First, there is no universal definition for advanced paternal ageing. Secondly, the literature is full of studies with conflicting results, especially for the most common parameters tested. Advancing paternal age also has been associated with increased risk of genetic disease. Our exhaustive literature review has demonstrated negative effects on sperm quality and testicular functions with increasing paternal age. Epigenetics changes, DNA mutations along with chromosomal aneuploidies have been associated with increasing paternal age. In addition to increased risk of male infertility, paternal age has also been demonstrated to impact reproductive and fertility outcomes including a decrease in IVF/ICSI success rate and increasing rate of preterm birth. Increasing paternal age has shown to increase the incidence of different types of disorders like autism, schizophrenia, bipolar disorders, and childhood leukemia in the progeny. It is thereby essential to educate the infertile couples on the disturbing links between increased paternal age and rising disorders in their offspring, to better counsel them during their reproductive years.

Telomere distribution pattern and synapsis initiation during spermatogenesis in zebrafish

BACKGROUND

Telomeres are located at ends of eukaryotic chromosomes and can affect proper chromosomal positioning. During spermatogenesis, the appropriate dynamics and behavior of chromosomes is crucial to generate haploid cells through meiosis. Here, we describe telomere distribution patterns during spermatogenesis in zebrafish, especially during meiotic prophase I, using fluorescence in situ hybridization. This was combined with synaptonemal complex protein 3 immunostaining, which allows the staging of spermatocytes.

RESULTS

During spermatogonial proliferation and the preleptotene stage, telomeres were dispersed throughout the nucleus. During the leptotene stage, telomeres temporarily moved to one pole of the nucleus at which γ-tubulin was located, forming the telomere bouquet. The cluster lasted until the onset of zygotene where it coincided with terminal synapsis initiation. They then spread around the periphery of the nucleus during the zygotene to pachytene stages. During postmeiotic stages, telomeres in spermatids and sperm were again dispersed throughout the nuclei. Application of this procedure in meiotic mutants confirmed that meiotic telomere clustering is independent of axial element formation of the synaptonemal complex.

CONCLUSIONS

These data clearly showed the clustering and distributions of telomeres throughout spermatogenesis in zebrafish. This procedure could be used to screen for mutants that have primary defects in telomere clustering.

A clinical appraisal of the genetic basis in unexplained male infertility

Unexplained male infertility (UMI), the inability to reproduce despite having a normal sexual history, physical exam and semen analysis, can have a genetic origin. Currently, few diagnostic tools are available for detecting such genetic abnormalities. Karyotyping and fluorescence in situ hybridization (FISH) are respectively used for chromosomal alterations in somatic cells and sperm aneuploidy assessment. Gene sequencing and mutational analysis have been introduced for identifying specific mutations and polymorphisms. Other approaches to the molecular evaluation of spermatozoa are under investigation, including array comparative genomic hybridization and whole-genome sequencing and non-coding ribonucleic acid arrays. Although treating cytogenetic abnormalities and genetic aberrations is still out of reach, the integration of these novel techniques may unravel hidden genetic defects in UMI. Finally, a deeper understanding of the sperm epigenome might allow the development of therapies based on epigenome modifications. This review focuses on the genetic basis of UMI and highlights the current and future methods for the evaluation of genetic defects as they relate to UMI. Review of the literature was carried out using ScienceDirect, OVID, PubMed and MedLine search engines.

A mutation in the H/ACA box of telomerase RNA component gene (TERC) in a young patient with myelodysplastic syndrome

Background

Telomeres are repeated sequences (the hexanucleotide TTAGGG in vertebrates) located at chromosome ends of eukaryotes, protecting DNA from end joining or degradation. Telomeres become shorter with each cell cycle, but telomerase, a ribonucleoprotein complex, alleviates this attrition. The telomerase RNA component (TERC) is an essential element of telomerase, serving as a template for telomere elongation. The H/ACA domain of TERC is indispensable for telomere biogenesis. Mutations in the telomerase components allow accelerated telomere loss, resulting in various disease manifestations, including bone marrow failure. To date, this is the first detailed report of an H-box mutation in TERC that is related to human disease.

Case presentation

A 26-year-old man with myelodysplastic syndrome (MDS) had very short telomeres. Sequencing identified a single heterozygous mutation in the H box of the patient’s TERC gene. The same mutation was also present in his father and his son, demonstrating that it was germline in origin. The telomere length in the father’s blood was shorter compared to age-matched healthy controls, while it was normal in the son and also in the sperm cells of the patient. In vitro experiments suggested that the mutation was responsible for the telomere shortening in the patient’s leukocytes and contributed to the pathogenesis of bone marrow failure in our patient.

Conclusion

We analyzed a mutation (A377G) in the H box of TERC in a young MDS patient who had significantly short-for-age telomeres. As telomeres protect chromosomes from instability, it is highly plausible that this genetic lesion was responsible for the patient’s hematological manifestations, including marrow failure and aneuploidy in the hematopoietic stem cell compartment.

Telomere-independent ageing in the longest-lived non-colonial animal, Arctica islandica

The shortening of telomeres as a causative factor in ageing is a widely discussed hypothesis in ageing research. The study of telomere length and its regenerating enzyme telomerase in the longest-lived non-colonial animal on earth, Arctica islandica, should inform whether the maintenance of telomere length plays a role in reaching the extreme maximum lifespan (MLSP) of >500years in this species. Since longitudinal measurements on living animals cannot be achieved, a cross-sectional analysis of a short-lived (MLSP 40years from the Baltic Sea) and a long-lived population (MLSP 226years Northeast of Iceland) and in different tissues of young and old animals from the Irish Sea was performed. A high heterogeneity of telomere length was observed in investigated A. islandica over a wide age range (10-36years for the Baltic Sea, 11-194years for Irish Sea, 6-226years for Iceland). Constant telomerase activity and telomere lengths were detected at any age and in different tissues; neither correlated with age or population habitat. Stable telomere maintenance might contribute to the long lifespan of A. islandica. Telomere dynamics are no explanation for the distinct MLSPs of the examined populations and thus the cause of it remains to be investigated.

Alteration of poly(ADP-ribose) metabolism affects murine sperm nuclear architecture by impairing pericentric heterochromatin condensation

The mammalian sperm nucleus is characterized by unique properties that are important for fertilization. Sperm DNA retains only small numbers of histones in distinct positions, and the majority of the genome is protamine associated, which allows for extreme condensation and protection of the genetic material. Furthermore, sperm nuclei display a highly ordered architecture that is characterized by a centrally located chromocenter comprising the pericentromeric chromosome regions and peripherally positioned telomeres. Establishment of this unique and well-conserved nuclear organization during spermiogenesis is not well understood. Utilizing fluorescence in situ hybridization (FISH), we show that a large fraction of the histone-associated sperm genome is repetitive in nature, while a smaller fraction is associated with unique DNA sequences. Coordinated activity of poly(ADP-ribose) (PAR) polymerase and topoisomerase II beta has been shown to facilitate DNA relaxation and histone to protamine transition during spermatid condensation, and altered PAR metabolism is associated with an increase in sperm histone content. Combining FISH with three-dimensional laser scanning microscopy technology, we further show that altered PAR metabolism by genetic or pharmacological intervention leads to a disturbance of the overall sperm nuclear architecture with a lower degree of organization and condensation of the chromocenters formed by chromosomal pericentromeric heterochromatin.

Sperm flow cytometric parameters are associated with ICSI outcome

The association of sperm nuclear chromatin condensation and ploidy with embryo development and outcome after intracytoplasmic sperm injection (ICSI) was explored. The study population consisted of 16 couples referred to Ioannina University Medical School In vitro Fertilization Unit with male factor infertility and serious impairments in sperm nuclear chromatin condensation and ploidy, according to sperm flow cytometry. Additionally, 20 couples with male factor infertility and relatively high sperm flow cytometry parameters participated as controls. The 35 cycles of the study population were characterized by a lower fertilization rate (P<0.001) as well as decreased grade A embryo rate (P=0.004) and increased grade C embryo rate (P=0.028), compared with the 29 cycles of the control group. Additionally, a significantly elevated arrested embryo rate (P<0.001) and a decreased clinical pregnancy rate (P<0.020) were observed in the couples of the study population. Consequently, high levels of sperm nuclear chromatin condensation abnormalities and sperm aneuploidies are probably associated with lower fertilization rates, impaired embryo quality, elevated arrested embryo rates and decreased pregnancy rates. These preliminary results strongly support the use of sperm flow cytometry as a potential prognostic tool of ICSI outcome.

Delayed paternal age of reproduction in humans is associated with longer telomeres across two generations of descendants

Telomeres are repeating DNA sequences at the ends of chromosomes that protect and buffer genes from nucleotide loss as cells divide. Telomere length (TL) shortens with age in most proliferating tissues, limiting cell division and thereby contributing to senescence. However, TL increases with age in sperm, and, correspondingly, offspring of older fathers inherit longer telomeres. Using data and samples from a longitudinal study from the Philippines, we first replicate the finding that paternal age at birth is associated with longer TL in offspring (n = 2,023, P = 1.84 × 10(-6)). We then show that this association of paternal age with offspring TL is cumulative across multiple generations: in this sample, grandchildren of older paternal grandfathers at the birth of fathers have longer telomeres (n = 234, P = 0.038), independent of, and additive to, the association of their father's age at birth with TL. The lengthening of telomeres predicted by each year that the father's or grandfather's reproduction are delayed is equal to the yearly shortening of TL seen in middle-age to elderly women in this sample, pointing to potentially important impacts on health and the pace of senescent decline in tissues and systems that are cell-replication dependent. This finding suggests a mechanism by which humans could extend late-life function as average age at reproduction is delayed within a lineage.

Specialization of a Drosophila capping protein essential for the protection of sperm telomeres

BACKGROUND

A critical function of telomeres is to prevent fusion of chromosome ends by the DNA repair machinery. In Drosophila somatic cells, assembly of the protecting capping complex at telomeres notably involves the recruitment of HOAP, HP1, and their recently identified partner, HipHop. We previously showed that the hiphop gene was duplicated before the radiation of the melanogaster subgroup of species, giving birth to K81, a unique paternal effect gene specifically expressed in the male germline.

RESULTS

Here we show that K81 specifically associates with telomeres during spermiogenesis, along with HOAP and HP1, and is retained on paternal chromosomes until zygote formation. In K81 mutant testes, capping proteins are not maintained at telomeres in differentiating spermatids, resulting in the transmission of uncapped paternal chromosomes that fail to properly divide during the first zygotic mitosis. Despite the apparent similar capping roles of K81 and HipHop in their respective domain of expression, we demonstrate by in vivo reciprocal complementation analyses that they are not interchangeable. Strikingly, HipHop appeared to be unable to maintain capping proteins at telomeres during the global chromatin remodeling of spermatid nuclei.

CONCLUSIONS

Our data demonstrate that K81 is essential for the maintenance of capping proteins at telomeres in postmeiotic male germ cells. In species of the melanogaster subgroup, HipHop and K81 have not only acquired complementary expression domains, they have also functionally diverged following the gene duplication event. We propose that K81 specialized in the maintenance of telomere protection in the highly peculiar chromatin environment of differentiating male gametes.

Green tea extracts attenuate doxorubicin-induced spermatogenic disorders in conjunction with higher telomerase activity in mice

PURPOSE

The aim of this study was to investigate the protective effect of green tea extracts against doxorubicin-induced damage in the mouse testes correlating with telomerase activity.

METHODS

Green tea extracts were administered orally. Doxorubicin was coadministered intraperitoneally. These testes were evaluated histologically and the telomerase activity was analyzed. Additional immunostaining was carried out.

RESULTS

Both the sperm density and sperm motility were significantly increased in green tea extracts coadministration groups as compared to the doxorubicin-treated groups. By histological analysis, germ cell damage was greatly attenuated by green tea extracts coadministration. Telomerase activity significantly increased in association with the coadministration of green tea extracts as compared to that of doxorubicin-only groups. In all groups, human telomerase reverse transcriptase signals were mainly observed in the spermatocytes and spermatids.

CONCLUSIONS

These findings suggest that green tea extracts exert protective effects against doxorubicin-induced spermatogenic disorders in conjunction with higher telomerase activity levels.

The genetic causes of male factor infertility: a review

OBJECTIVE

To illustrate the necessity for an enhanced understanding of the genetic basis of male factor infertility, to present a comprehensive synopsis of these genetic elements, and to review techniques being utilized to produce new insights in fertility research.

BACKGROUND

Male factor infertility is a complex disorder that affects a large sector of the population; however, many of its etiologies are unknown. By elucidating the underlying genetic basis of infertile phenotypes, it may be possible to discover the causes of infertility and determine effective treatments for patients.

METHOD(S)

The PubMed database was consulted for the most relevant papers published in the last 3 years pertaining to male factor infertility using the keywords "genetics" and "male infertility."

RESULT(S)

Advances have been made in the characterization of the roles of specific genes, but further research is necessary before these results can be used as guidelines for diagnosing and treating male factor infertility. The accurate transmission of epigenetic information also has considerable influence on fertility in males and on the fertility of their offspring.

CONCLUSION(S)

Analysis of the genetic factors that impact male factor infertility will provide valuable insights into the creation of targeted treatments for patients and the determination of the causes of idiopathic infertility. Novel technologies that analyze the influence of genetics from a global perspective may lead to further developments in the understanding of the etiology of male factor infertility through the identification of specific infertile phenotype signatures.

H2A.Bbd: an X-chromosome-encoded histone involved in mammalian spermiogenesis

Despite the identification of H2A.Bbd as a new vertebrate-specific replacement histone variant several years ago, and despite the many in vitro structural characterizations using reconstituted chromatin complexes consisting of this variant, the existence of H2A.Bbd in the cell and its location has remained elusive. Here, we report that the native form of this variant is present in highly advanced spermiogenic fractions of mammalian testis at the time when histones are highly acetylated and being replaced by protamines. It is also present in the nucleosomal chromatin fraction of mature human sperm. The ectopically expressed non-tagged version of the protein is associated with micrococcal nuclease-refractory insoluble fractions of chromatin and in mouse (20T1/2) cell line, H2A.Bbd is enriched at the periphery of chromocenters. The exceedingly rapid evolution of this unique X-chromosome-linked histone variant is shared with other reproductive proteins including those associated with chromatin in the mature sperm (protamines) of many vertebrates. This common rate of evolution provides further support for the functional and structural involvement of this protein in male gametogenesis in mammals.

Free radical theory of aging: implications in male infertility

This review examines the effect of mitochondrial generation of reactive oxygen species (ROS) and aging on human spermatozoa and seminal antioxidants. We discuss the effect of continuous ROS production on biomarkers of aging, such as germ cell telomeres and telomerase, lipofuscin, and amyloid. These markers may be responsible for telomere shortening and subsequent decrease in sperm count, decline in testosterone concentration, and decline in motility with aging. Excessive ROS can also damage mitochondrial deoxyribonucleic acid and sperm nuclear DNA, contributing to paternally transmitted diseases. ROS generation has a central role in the pathophysiology of age-related decrease in male fertility.

Reorganisation of human sperm nuclear architecture during formation of pronuclei in a model system

By fertilisation, two terminally differentiated cells, namely the egg and spermatozoon, are combined to create a totipotent zygote. During this process, the inactive sperm nucleus is transformed into a functional male pronucleus. Recent studies demonstrate that human sperm chromatin has an elaborate multilevel organisation, but almost nothing is known about how sperm chromosomes are transformed during fertilisation. Because of ethical reasons and technical complications, experimentation with human embryos is generally unworkable and adequate model systems are necessary to study the formation of male pronuclei. Here, we analyse remodelling of human sperm chromatin and chromosome architecture in Xenopus egg extracts using immunofluorescent localisation of protamines and centromere protein A, as well as fluorescence in situ hybridisation localisation of major α-satellite DNA and whole chromosome territory (CT). We demonstrate noticeable relocalisation of centromeres and remodelling of CT during the decondensation–recondensation cycle, mimicking cellular events that occur in the paternal genome in vivo during fertilisation.

Positioning of chromosome 15, 18, X and Y centromeres in sperm cells of fertile individuals and infertile patients with increased level of aneuploidy

Evidence has been accumulating that individual chromosomes in human sperm cells occupy defined, non-random positions. Our earlier study suggested that abnormal spermatogenesis in carriers of reciprocal translocations was reflected in the changes in the intranuclear topology of sperm chromosomes. The purpose of this study was to determine whether the increased level of disomy of sperm chromosomes may be the factor that can disturb topology within the sperm nuclei. The results obtained indicated that within the sperm nuclei of fertile individuals the centromeres of chromosomes 15, 18, X and Y were localized in a small area that may be a fragment of the chromocentre. When compared with the intranuclear positions of the same chromosomes in sperm nuclei of infertile patients with an increased level of aneuploidy, some disturbances in the centromere area were found. In disomic sperm cells (n + 1) centromeres 15,15 or 18,18 or YY (but not X,X) had a shifted average longitudinal position in comparison with normal sperm cells (n = 23).

Epigenetic reprogramming of the male genome during gametogenesis and in the zygote

During post-meiotic maturation, male germ cells undergo a formidable reorganization and condensation of their genome. During this phase most histones are globally acetylated and then replaced by sperm-specific basic proteins, named protamines, which compact the genome into a very specific structure within the sperm nucleus. Several studies suggest that this sperm-specific genome packaging structure conveys an important epigenetic message to the embryo. This paper reviews what is known about this fundamental, yet poorly understood, process, which involves not only global changes of the structure of the haploid genome, but also localized specific modifications of particular genomic regions, including pericentric heterochromatin and sex chromosomes. After fertilization, the male genome undergoes a drastic decondensation, and rapidly incorporates new histones. However, it remains different from the maternal genome, bearing specific epigenetic marks, especially in the pericentric heterochromatin region. The functional implications of male post-meiotic and post-fertilization genome reprogramming are not well known, but there is experimental evidence to show that it affects early embryonic development.

Long-term effects of mouse intracytoplasmic sperm injection with DNA-fragmented sperm on health and behavior of adult offspring

Genetic and environmental factors produce different levels of DNA damage in spermatozoa. Usually, DNA-fragmented spermatozoa (DFS) are used with intracytoplasmic sperm injection (ICSI) treatments in human reproduction, and use of DFS is still a matter of concern. The purpose of the present study was to investigate the long-term consequences on development and behavior of mice generated by ICSI with DFS. Using CD1 and B6D2F1 mouse strains, oocytes were injected with fresh spermatozoa or with frozen-thawed spermatozoa without cryoprotector. This treatment increased the percentage of TUNEL-positive spermatozoa, tail length as measured by comet assay, and loss of telomeres as measured by quantitative PCR. The ICSI-generated embryos were cultured for 24 h in KSOM, and 2-cell embryos were transferred into CD1 females. The DFS reduced both the rate of preimplantation embryo development and number of offspring. Immunofluorescence staining with an antibody against 5-methylcytosine showed a delay of 2 h on the active demethylation of male pronucleus in the embryos produced by ICSI. Moreover, ICSI affected gene transcription and methylation of some epigenetically regulated genes like imprinting, X-linked genes, and retrotransposon genes. At 3 and 12 mo of age, ICSI with DFS-produced animals and in vivo-fertilized controls were submitted to behavioral tests: locomotor activity (open field), exploratory/anxiety behavior (elevated plus maze, open field), and spatial memory (free-choice exploration paradigm in Y maze). Females produced by ICSI showed increased anxiety, lack of habituation pattern, deficit in short-term spatial memory, and age-dependent hypolocomotion in the open-field test (P<0.05). Postnatal weight gain of mice produced by ICSI with fresh or frozen sperm was higher than that of their control counterparts from 16 wk on (P<0.01). Anatomopathological analysis of animals at 16 mo of age showed some large organs and an increase in pathologies (33% of CD1 females produced with DFS presented some solid tumors in lungs and dermis of back or neck). Moreover, 20% of the B6D2F1 mice generated with DFS died during the first 5 mo of life, with 25% of the surviving animals showing premature aging symptoms, and 70% of the B6D2F1 mice generated with DFS died earlier than controls with different kind of tumors. We propose that depending on the level of DFS, oocytes may partially repair fragmented DNA, producing blastocysts able to implant and produce live offspring. The incomplete repair, however, may lead to long-term pathologies. Our data indicate that use of DFS in ICSI can generate effects that only emerge during later life, such as aberrant growth, premature aging, abnormal behavior, and mesenchymal tumors.

The effect of epigenetic sperm abnormalities on early embryogenesis

Sperm are a highly specialized cell type derived to deliver the paternal haploid genome to the oocyte. The epigenetic, or gene regulatory, properties and mechanisms of the sperm assist in preparation of the paternal genome to contribute to embryogenesis and the genome of the zygote. Many recent studies have addressed the issue of altered epigenetic processes in the sperm. This review evaluates the current understanding of DNA damage, chromosome aneuploidy, reduced telomere length, malformations of the centrosome, genomic imprinting errors, altered mRNA profiles, and abnormal nuclear packaging in the sperm prior to fertilization and the observed effects on embryogenesis. Attention has also been given to understanding the underlying etiology of sperm with altered epigenetic mechanisms in humans.

Protamines and male infertility

Protamines are the major nuclear sperm proteins. The human sperm nucleus contains two types of protamine: protamine 1 (P1) encoded by a single-copy gene and the family of protamine 2 (P2) proteins (P2, P3 and P4), all also encoded by a single gene that is transcribed and translated into a precursor protein. The protamines were discovered more than a century ago, but their function is not yet fully understood. In fact, different hypotheses have been proposed: condensation of the sperm nucleus into a compact hydrodynamic shape, protection of the genetic message delivered by the spermatozoa, involvement in the processes maintaining the integrity and repair of DNA during or after the nucleohistone-nucleoprotamine transition and involvement in the epigenetic imprinting of the spermatozoa. Protamines are also one of the most variable proteins found in nature, with data supporting a positive Darwinian selection. Changes in the expression of P1 and P2 protamines have been found to be associated with infertility in man. Mutations in the protamine genes have also been found in some infertile patients. Transgenic mice defective in the expression of protamines also present several structural defects in the sperm nucleus and have variable degrees of infertility. There is also evidence that altered levels of protamines may result in an increased susceptibility to injury in the spermatozoan DNA causing infertility or poor outcomes in assisted reproduction. The present work reviews the articles published to date on the relationship between protamines and infertility.

Chromosome architecture in the decondensing human sperm nucleus

Whereas recent studies demonstrated a well-defined nuclear architecture in human sperm nuclei, little is known about the mode of DNA compaction above the elementary structural unit of nucleoprotamine toroids. Here, using fluorescence in-situ hybridization (FISH) with arm-specific DNA probes of chromosomes 1, 2 and 5, we visualized arm domains and established hierarchical levels of sperm chromatin structures. The compact chromosome territories, which in sperm have a preferred intranuclear localization, have an extended conformation represented by a 2000 nm chromatin fiber. This fiber is composed of a 1000 nm chromatin thread bent at 180 degrees near centromere. Two threads of 1000 nm, representing p-arm and q-arm chromatin, run in antiparallel fashion and join at the telomeres. Each 1000 nm thread, in turn, resolves into two rows of chromatin globules 500 nm in diameter interconnected with thinner chromatin strands. We propose a unified comprehensive model of chromosomal and nuclear architecture in human sperm that, as we suggest, is important for successful fertilization and early development.

Dynamic rearrangement of telomeres during spermatogenesis in mice

Chromosomal structure within the nucleus influences various biological processes such as transcription and replication. Telomeres are located at the end of eukaryotic chromosomes and they can be a decisive factor for correct chromosomal positioning. To gain new insight into telomere dynamics, we examined telomere length and positional changes during spermatogenesis using improved fluorescence in situ hybridization (FISH) and in situ telomeric repeat amplification protocols (TRAP) on histological sections. FISH revealed telomere length and chromosome position within nuclei change dynamically. Telomere extension occurred during spermiogenesis. In situ TRAP analysis verified elevated telomerase activity in elongating spermatids. Together, these data show that elongated spermatids have longer telomeres than precursor spermatogenic cells. This observation indicates that telomere elongation in haploid cells occurs after meiosis and in the absence of genomic replication. Analyses of testes from telomerase null mice further support the significance of telomere dynamics during spermatogenesis and the existence of an alternative telomere extension pathway.

Establishment of male-specific epigenetic information

The setting of male-specific epigenetic information is a complex process, which involves a major global re-organisation, as well as localized changes of the nucleus structure during the pre-meiotic, meiotic and post-meiotic stages of the male germ cell differentiation. Although it has long been known that DNA methylation in targeted regions of the genome is associated with male-specific genomic imprinting, or that most core histones are hyperacetylated and then replaced by sperm-specific proteins during the post-meiotic condensation of the nucleus, many questions remain unanswered. How these changes interact, how they affect the epigenetic information and how the paternal epigenetic marks contribute to the future genome are indeed major issues remaining to be explored.

Replicative senescence: a critical review

Human cells in culture have a limited proliferative capacity. After a period of vigorous proliferation, the rate of cell division declines and a number of changes occur in the cells including increases in size, in secondary lysosomes and residual bodies, nuclear changes and a number of changes in gene expression which provide biomarkers for senescence. Although human cells in culture have been used for over 40 years as models for understanding the cellular basis of aging, the relationship of replicative senescence to aging of the organism is still not clear. In this review, we discuss replicative senescence in the light of current information on signal transduction and mitogenesis, cell stress, apoptosis, telomere changes and finally we discuss replicative senescence as a model of aging in vivo.

Irregular telomeres impair meiotic synapsis and recombination in mice

Telomere shortening can lead to chromosome instability, replicative senescence, and apoptosis in both somatic and male germ cells. To study roles for mammalian telomeres in homologous pairing and recombination, we characterized effects of telomere shortening on spermatogenesis and oogenesis in late-generation telomerase-deficient mice. We show that shortened telomeres of late-generation telomerase-deficient mice impair meiotic synapsis and decrease recombination, in particular, in females. In response to telomere shortening, male germ cells mostly undergo apoptosis, whereas female germ cells preferentially arrest in early meiosis, suggesting sexually dimorphic surveillance mechanisms for telomere dysfunction during meiosis in mice. Further, meiocytes of late-generation telomerase-deficient females with shortened telomeres, bred with early-generation males harboring relatively long telomeres, exhibit severely impaired chromosome pairing and synapsis and reduced meiotic recombination. These findings imply that functional telomeres are important in mammalian meiotic synapsis and recombination.