Telomere-telomere interactions and candidate telomere binding protein(s) in mammalian sperm cells

Chromatin structure of telomere domain in human sperm

Telomeres in human sperm nucleus are clustered at the nuclear periphery. Chromosomes in the sperm are highly condensed with protamines, however, a small portion of DNA remains associated with histones; the role of the nucleohistone is unknown. To examine structure of the telomeric chromatin, the sperm nuclei were treated with micrococcal nuclease. Chromatin released by the digestion was free from protamines, but contained histones and revealed nucleosomal organization. It was enriched with telomeric DNA organized into closely spaced nucleosomes with a periodicity of 148 +/- bp. Thus, while the most of the sperm genome is packed into extremely dense nucleoprotamine structure, at least a part of the telomeric DNA is arranged into nucleosomes and can be released by the nuclease. We suggest that telomeres might be among the first structures in the sperm nucleus that respond to oocyte signals for male pronucleus development at fertilization.

Mammalian meiotic telomeres: protein composition and redistribution in relation to nuclear pores

Mammalian telomeres consist of TTAGGG repeats, telomeric repeat binding factor (TRF), and other proteins, resulting in a protective structure at chromosome ends. Although structure and function of the somatic telomeric complex has been elucidated in some detail, the protein composition of mammalian meiotic telomeres is undetermined. Here we show, by indirect immunofluorescence (IF), that the meiotic telomere complex is similar to its somatic counterpart and contains significant amounts of TRF1, TRF2, and hRap1, while tankyrase, a poly-(ADP-ribose)polymerase at somatic telomeres and nuclear pores, forms small signals at ends of human meiotic chromosome cores. Analysis of rodent spermatocytes reveals Trf1 at mouse, TRF2 at rat, and mammalian Rap1 at meiotic telomeres of both rodents. Moreover, we demonstrate that telomere repositioning during meiotic prophase occurs in sectors of the nuclear envelope that are distinct from nuclear pore-dense areas. The latter form during preleptotene/leptotene and are present during entire prophase I.

Chromosome-specific telomeric associations in Chinese hamster embryonic cells

Telomeric associations (TAs) represent an important cytogenetic marker of human tumor cells. It has been thought that the primary cause of TAs is telomere shortening. However, we report here a surprising aspect of telomere maintenance in primary Chinese hamster embryonic (CHE) cells: relatively high frequencies of TAs in spite of normal telomere length. These TAs are present in both interphase and metaphase cells, suggesting that metaphase TAs may be relics of interphase chromosome organization. In addition, some TAs observed here are chromosome-specific and recurrent in at least three consecutive cell cycles in two different CHE cell strains. In spite of relatively high frequencies of TAs, none of the CHE strains show chromosome instability resulting from breakage-fusion-bridge cycles, as would be expected from tumor cell studies. It appears that TAs in CHE cells may be reversible events. These results are discussed in light of current understanding of telomere biology.

Genome organization in the human sperm nucleus studied by FISH and confocal microscopy

The sperm nucleus has a unique chromatin structure where the DNA is highly condensed and associated with specific proteins, the protamines. It is a nondividing cell which is also transcriptionally inactive. After fusion with an oocyte, the sperm nucleus undergoes decondensation and, in the same time, starts replication and transcription. It has been suggested that somatic chromosomes during interphase are organized in territories which display a cell type and cell cycle specific distribution. The purpose of this work was to investigate whether chromosomes would also have a specific distribution in the sperm nucleus, which could be related to its inactive state, and have implications on the early stages of fertilization. In the present study, centromeric and telomeric sequences were detected by fluorescent techniques performed on human decondensed spermatozoa. Chromosome painting probes were used to detect the chromosome X and chromosome 13 on interphase sperm nuclei. The fluorescent signals were captured in 3D with a confocal microscope. For each of these chromatin structures, the volume, position, and distribution of the signals were analyzed in samples of 30 nuclei with the help of image analysis software. The centromeres appeared grouped in several foci that were randomly distributed within the sperm nucleus. The telomeres gave an approximately haploid number of small signals, evenly distributed throughout the nucleus. The chromosomes X and 13 occupied 4.7% and 3. 7% of the total nuclear volume, respectively. Interestingly, the X chromosome territory showed a preferential position in the anterior half of the volume of the nucleus, whereas chromosome 13 had a random position. This work shows a particular distribution of chromosome territories in the human sperm nucleus that could be related to mechanisms implicated in its specific functions. The analysis of more chromosomes and chromosomal structures, including the Y chromosome, would help to understand the structure of the human sperm chromatin, and its fundamental and clinical implications.

Identification and characterization of a bovine sperm protein that binds specifically to single-stranded telomeric deoxyribonucleic acid

Telomere DNA at the physical termini of chromosomes forms a single-stranded 3' overhang. In lower eukaryotes, e.g., ciliated protozoa, this DNA extension is capped by specific proteins that have been structurally and functionally characterized. Much less is known about single-stranded telomere DNA-binding proteins in vertebrates. Here we describe a new protein from bovine sperm designated bsSSTBP that specifically interacts with single-stranded (TTAGGG)(N) DNA. The bsSSTBP was extracted from nuclei by 0.6 M KCl. The native size of this protein, estimated by gel filtration, was 20-40 kDa. SDS-PAGE of the UV cross-linked complex between bsSSTBP and telomere DNA indicated that several polypeptides are involved in complex formation. Bovine sSSTB had high specificity toward nucleotide sequence, since single nucleotide substitutions in the (TTAGGG)(4) substrate suppressed binding. The minimal number of (TTAGGG) repeats required for binding of bsSSTBP was 3, and the protein recognized linear but not folded DNA structures. We propose that the bsSSTBP participates in telomere-telomere interactions and the telomere membrane localization observed in mature sperm. In mammals, somatic telomere-binding proteins are apparently substituted by sperm-specific ones that may lead to a structural reorganization of telomere domains to fulfill functions important during meiosis and fertilization.

Sperm nuclear activation during fertilization

The delivery of the paternal genome to the egg is a primary goal of fertilization. In preparation for this step, the nucleus of the developing spermatozoon undergoes extensive morphological and biochemical transformations during spermatogenesis to yield a tightly compacted sperm nucleus. These modifications are essentially reversed during fertilization. As a result, the incorporated sperm nucleus undergoes many steps in the egg cytoplasm as it develops into a male pronucleus. The sperm nucleus (1) loses its nuclear envelope, (2) undergoes nucleoprotein remodeling, (3) decondenses and increases in size, (4) becomes more spherical, (5) acquires a new nuclear envelope, and (6) becomes functionally competent to synthesize DNA and RNA. These changes are coordinate with meiotic processing of the maternal chromatin, and often result in behaviors asynchronous with the maternal chromatin. For example, in eggs fertilized during meiosis, the sperm nucleus decondenses while the maternal chromatin remains condensed. A model is presented that suggests some reasons why this puzzling behavior exists. Defects in any of the processes attending male pronuclear development often result in infertility. New assisted reproductive technologies have been developed that ensure delivery of the sperm nucleus to the egg cytoplasm so that a healthy embryo is produced. An emerging challenge is to further characterize the molecular mechanisms that control sperm nuclear transformations and link these to causes of human infertility. Further understanding of this basic process promises to revolutionize our understanding of the mystery of the beginning of new life.

Gbp1p, a protein with RNA recognition motifs, binds single-stranded telomeric DNA and changes its binding specificity upon dimerization

Gbp1p is a putative telomere-binding protein from Chlamydomonas reinhardtii that contains two RNA recognition motifs (RRMs) which are commonly found in heterogeneous nuclear ribonucleoproteins (hnRNPs). Previously we demonstrated that Gbp1p binds single-stranded DNA (ssDNA) containing the Chlamydomonas telomeric sequence but not the RNA containing the cognate sequence. Here we show that at lower protein concentrations Gbp1 can also bind an RNA containing the cognate sequence. We found that mutation of the two RRM motifs of Gbp1p to match the highly conserved region of hnRNP RRMs did not alter the affinity of Gbp1p for either RNA or DNA. The ability of Gbp1p to associate with either of these two nucleic acids is governed by the dimerization state of the protein. Monomeric Gbp1p associates with either ssDNA or RNA, showing a small binding preference for RNA. Dimeric Gbp1p has a strong preference for binding ssDNA and shows little affinity for RNA. To the best of our knowledge, this is the first example of a protein that qualitatively shifts its nucleic acid binding preference upon dimerization. The biological implications of a telomere-binding protein that is regulated by dimerization are discussed.

Increased telomere size in sperm cells of mammals with long terminal (TTAGGG)n arrays

An increase in the length of telomeres in human sperm compared to somatic cells has long been noted and considered within a popular hypothesis involving telomere shortening and cell aging. In the present study we determined telomere length in two species with long terminal TTAGGG arrays--bovine and porcine. Using several independent methods we demonstrate that the telomeres in the sperm of human, porcine and bovine are elongated by 69%, 24%, and 14%, respectively, in comparison with somatic tissues. Therefore, increased sperm telomere length is a feature preserved throughout mammalian evolution. The biological role of this phenomenon is discussed in the context of telomere functions in meiosis and fertilization.

Aspects of three-dimensional chromosome reorganization during the onset of human male meiotic prophase

The three-dimensional morphology and distribution of human chromosomes 3 were studied in nuclei of spermatogonia and spermatocytes I from formaldehyde-fixed human testis sections. Chromosome arms, pericentromeres and telomeric regions were painted by a three-color, five-probe fluorescence in situ hybridization protocol. Light optical serial sections of premeiotic and meiotic nuclei obtained by confocal laser scanning microscopy revealed that premeiotic chromosomes 3 are separate from each other and occupy variably shaped territories, which are sectored in distinct 3 p- and q-arm domains. Three-dimensional reconstructions of the painted chromosome domains by a Voronoi tessellation approach showed that mean chromosome volumes did not differ significantly among the premeiotic and meiotic stages investigated. A significant increase in surface area and reduction of dimensionless ‘roundness factor’ estimates of arm domains indicated that the restructuring of spatially separate chromosome territories initiates during preleptotene. Telomeric regions, which in meiotic stem cells located predominantly in arm-domain chromatin, showed a redistribution towards the domain surface during this stage. At leptotene homologues were generally misaligned and displayed intimate intermingling of non-homologous chromatin. Pairing initiated at the ends of bent zygotene chromosomes, which displayed a complex surface structure with discernible sister chromatids. The results indicate that, in mammals, homology search is executed during leptotene, after remodeling of chromosome territories.

Nuclear envelope remodelling during rat spermiogenesis: distribution and expression pattern of LAP2/thymopoietins

Lamina-associated polypeptide 2 (LAP2) and the thymopoietins (TPs) are a family of proteins described in somatic cells of mammals, which are derived by alternative splicing from a single gene. For one of the members of the family (LAP2 = TPbeta) it has been shown that this integral membrane protein locates to the inner membrane of the nuclear envelope, and that it binds to chromatin and B-type lamins. In the present study, we observed that during the third phase of spermatogenesis (i.e. spermiogenesis), TP-labelling shifted progressively to one half of the nuclear periphery in round spermatids. In the elongating spermatid the signal then becomes restricted to one spot located at the posterior (centriolar) pole of the nucleus. Changes in localization are accompanied by the disappearance, first of TPgamma, and later on of LAP2/TPbeta. TPalpha is the only member of the family detectable in the mature sperm. Concomitantly, lamin B1, the only nuclear lamina protein known to be expressed in mammalian spermatids, showed a similar behaviour, i.e. shifted progressively to the centriolar pole of spermatid nuclei before it became undetectable in fully differentiated mature sperms. These results are the first demonstration that expression and localization patterns of TPs are coordinately and differentially regulated with lamins during a differentiation process.

Clustering of pericentromeres initiates in step 9 of spermiogenesis of the rat (Rattus norvegicus) and contributes to a well defined genome architecture in the sperm nucleus

Fluorescence in situ hybridization with centromeric, telomeric and whole chromosome paint probes was used to study nuclear topology in epididymal sperm as well as spermatids from testis tissue sections of the rat. Pericentromeric regions of 9 chromosomes of the rat (n=21) were labeled with a satellite I specific DNA probe. Pericentromeres showed few tandem associations in spermatids of steps 1–8 of spermiogenesis. At step 9, pericentromeric regions associated to form an elongated cluster in the spermatid nucleus. This arrangement was also seen in the sperm nucleus. FISH with telomere probes revealed numerous, variably arranged signals in round and elongated spermatids as well as sperm nuclei. Telomere signals showed a tendency for pairwise association which was more pronounced in elongated spermatid and epididymal sperm nuclei. FISH to DTT treated sperm suggested that telomeres reside at the periphery and that pericentromeres are located in the nuclear interior. Chromosome painting with rat chromosome 2 and 12 specific microdissection library probes showed that these chromosomes predominantly occupy compact and variably shaped territories during spermatid maturation. In elongated epididymal sperm nuclei chromosome 2 and 12 territories took up specific positions. We suppose that the associations of pericentromeres during step 9 render a well defined nuclear topology which facilitates the ordered compaction of the genome at subsequent stages.