cDNA sequence of the murine synaptonemal complex protein 1 (SCP1)

In vitro germ cell differentiation from cynomolgus monkey embryonic stem cells

Background

Mouse embryonic stem (ES) cells can differentiate into female and male germ cells in vitro. Primate ES cells can also differentiate into immature germ cells in vitro. However, little is known about the differentiation markers and culture conditions for in vitro germ cell differentiation from ES cells in primates. Monkey ES cells are thus considered to be a useful model to study primate gametogenesis in vitro. Therefore, in order to obtain further information on germ cell differentiation from primate ES cells, this study examined the ability of cynomolgus monkey ES cells to differentiate into germ cells in vitro.

Methods and Findings

To explore the differentiation markers for detecting germ cells differentiated from ES cells, the expression of various germ cell marker genes was examined in tissues and ES cells of the cynomolgus monkey (Macaca fascicularis). VASA is a valuable gene for the detection of germ cells differentiated from ES cells. An increase of VASA expression was observed when differentiation was induced in ES cells via embryoid body (EB) formation. In addition, the expression of other germ cell markers, such as NANOS and PIWIL1 genes, was also up-regulated as the EB differentiation progressed. Immunocytochemistry identified the cells expressing stage-specific embryonic antigen (SSEA) 1, OCT-4, and VASA proteins in the EBs. These cells were detected in the peripheral region of the EBs as specific cell populations, such as SSEA1-positive, OCT-4-positive cells, OCT-4-positive, VASA-positive cells, and OCT-4-negative, VASA-positive cells. Thereafter, the effect of mouse gonadal cell-conditioned medium and growth factors on germ cell differentiation from monkey ES cells was examined, and this revealed that the addition of BMP4 to differentiating ES cells increased the expression of SCP1, a meiotic marker gene.

Conclusion

VASA is a valuable gene for the detection of germ cells differentiated from ES cells in monkeys, and the identification and characterization of germ cells derived from ES cells are possible by using reported germ cell markers in vivo, including SSEA1, OCT-4, and VASA, in vitro as well as in vivo. These findings are thus considered to help elucidate the germ cell developmental process in primates.

Regulating double-stranded DNA break repair towards crossover or non-crossover during mammalian meiosis

During meiosis the programmed induction of DNA double-stranded breaks (DSB) leads to crossover (CO) and non-crossover products (NCO). One key role of CO is to connect homologs before metaphase I and thus to ensure the proper reductional segregation. This role implies an accurate regulation of CO frequency with the establishment of at least one CO per chromosome arm. Current major challenges are to understand how CO and NCO formation are regulated and what is the role of NCO. We present here the current knowledge about CO and NCO and their regulation in mammals. CO density varies widely along chromosomes and their distribution is not random as they are subject to positive interference. As documented in the mouse and human, a significant excess of DSB are generated relative to the number of CO. In fact, evidence has been obtained for the formation of NCO products, for regulation of the choice of DSB repair towards CO or NCO and for a CO specific pathway. We discuss the roles of Msh4, Msh5 and Sycp1 which affect DSB repair and probably not only the CO pathway. We suggest that, in mammals, the regulation of NCO differs from that described in Saccharomyces cerevisiae.

NF-kappaB Activation Elicited by Ionizing Radiation Is Proapoptotic in Testis1

The transcription factor NF-kappaB modulates apoptotic machinery following activation by the IkappaB kinase (IKK) complex. Inhibiting activity of one of the catalytic subunits of the IKK complex, IKKbeta (also known as IKBKB and IKK2) severely inhibits NF-kappaB nuclear translocation in response to most stimuli, including ionizing radiation. Doubly floxed Ikkbeta(F/F) mice (control) were compared to haplo-insufficient Ikkbeta(F/)(delta) mice (NF-kappaB knockdown) to examine the in vivo apoptotic role of NF-kappaB in the testis. Although Ikkbeta(F/F) control adult mice had spermatid head counts and testis and body weights similar to Ikkbeta(F/)(delta) mice, cellular stress in the form of ionizing radiation elicited a differential phenotype. Lower body exposure to 5 Gy of ionizing radiation induced a greater NF-kappaB activation in Ikkbeta(F/F) than in Ikkbeta(F/)(delta) mice. In addition, exposure to ionizing radiation resulted in fewer apoptotic germ cells 3, 6, and 12 h after injury in NF-kappaB knockdown mice than in controls, concomitant with the reduced cleavage of caspases 3 and 9 at 3 h. There was also a reduction in total germ cells lost after radiation with NF-kappaB inhibition. Correspondingly, real-time RT-PCR showed a significant reduction in Cdnk1a (also known as p21) and Fasl expression 3 and 6 h, respectively, after irradiation in Ikkbeta(F/)(delta) compared to control testes. These data indicate that, despite acting in an antiapoptotic manner in many tissue types, NF-kappaB is proapoptotic in modulating the germ cell response to ionizing radiation.

PAIR2 is essential for homologous chromosome synapsis in rice meiosis I

The PAIR2 gene is required for homologous chromosome synapsis at meiosis I in rice (Oryza sativa L.) and encodes a HORMA-domain protein that is homologous to Saccharomyces cerevisiae HOP1 and Arabidopsis ASY1. Immunocytological and electron microscopic analyses indicate that PAIR2 proteins associate with axial elements (AEs) at leptotene and zygotene, and is removed from the AEs of arm regions when homologous chromosomes have been synapsed. Immunocytology against a centromeric histone H3 variant revealed that PAIR2 remains at centromeres until diakinesis, by which time the homologous centromeres had already been synapsed. However, neither precocious segregation of sister chromatids nor kinetochore dysfunction is observed, and AEs are normally assembled in the mutant. In the pair2-null mutant, homologous chromosome synapsis is completely eliminated. This study provides the first description of AE-associated protein in monocot plants and indicates that PAIR2 plays an essential role in promoting homologous chromosome synapsis. However, PAIR2 does not play a role in AE formation, sister chromatid cohesion at centromeres or kinetochore assembly in meiosis I of rice.

Mouse Sycp1 functions in synaptonemal complex assembly, meiotic recombination, and XY body formation

In meiotic prophase, synaptonemal complexes (SCs) closely appose homologous chromosomes (homologs) along their length. SCs are assembled from two axial elements (AEs), one along each homolog, which are connected by numerous transverse filaments (TFs). We disrupted the mouse gene encoding TF protein Sycp1 to analyze the role of TFs in meiotic chromosome behavior and recombination. Sycp1(-/-) mice are infertile, but otherwise healthy. Sycp1(-/-) spermatocytes form normal AEs, which align homologously, but do not synapse. Most Sycp1(-/-) spermatocytes arrest in pachynema, whereas a small proportion reaches diplonema, or, exceptionally, metaphase I. In leptotene Sycp1(-/-) spermatocytes, gammaH2AX (indicative of DNA damage, including double-strand breaks) appears normal. In pachynema, Sycp1(-/-) spermatocytes display a number of discrete gammaH2AX domains along each chromosome, whereas gammaH2AX disappears from autosomes in wild-type spermatocytes. RAD51/DMC1, RPA, and MSH4 foci (which mark early and intermediate steps in pairing/recombination) appear in similar numbers as in wild type, but do not all disappear, and MLH1 and MLH3 foci (which mark late steps in crossing over) are not formed. Crossovers were rare in metaphase I of Sycp1(-/-) mice. We propose that SYCP1 has a coordinating role, and ensures formation of crossovers. Unexpectedly, Sycp1(-/-) spermatocytes did not form XY bodies.

Mammalian protein SCP1 forms synaptonemal complex-like structures in the absence of meiotic chromosomes

Synaptonemal complexes (SCs) are evolutionary conserved, meiosis-specific structures that play a central role in synapsis of homologous chromosomes, chiasmata distribution, and chromosome segregation. However, it is still for the most part unclear how SCs do assemble during meiotic prophase. Major components of mammalian SCs are the meiosis-specific proteins SCP1, 2, and 3. To investigate the role of SCP1 in SC assembly, we expressed SCP1 in a heterologous system, i.e., in COS-7 cells that normally do not express SC proteins. Notably, under these experimental conditions SCP1 is able to form structures that closely resemble SCs (i.e., polycomplexes). Moreover, we show that mutations that modify the length of the central alpha-helical domain of SCP1 influence the width of polycomplexes. Finally, we demonstrate that deletions of the nonhelical N- or C-termini both affect polycomplex assembly, although in a different manner. We conclude that SCP1 is a primary determinant of SC assembly that plays a key role in synapsis of homologous chromosomes.

Cancer/testis-associated genes: identification, expression profile, and putative function

Cancer/testis-associated genes (CTAs) are a subgroup of tumor antigens with a restricted expression in testis and malignancies. During the last decade, many of these immunotherapy candidate genes have been discovered using various approaches. Most of these genes are localized on the X-chromosome, often as multigene families. Methylation status seems to be the main, but not the only regulator of their specific expression pattern. In testis, CTAs are exclusively present in cells of the germ cell lineage, though there is a lot of variation in the moment of expression during different stages of sperm development. Likewise, there is also a lot of heterogeneity in the expression of CTAs in melanoma samples. Clues regarding functionality of CTAs for many of these proteins point to a role in cell cycle regulation or transcriptional control. Better insights in the function of these genes may shed light on the link between spermatogenesis and tumor growth and could be of use in anti-tumor therapies. This review outlines the CTA family and focuses on their expression and putative function during male germ cell development and melanocytic tumor progression.

The mouse homolog of Drosophila Vasa is required for the development of male germ cells

Restricted expression of a mouse Vasa homolog gene (Mvh) expression is first detected in primordial germ cells (PGCs) after colonization of the genital ridges. Subsequently,Mvh is maintained until postmeiotic germ cells are formed. Here, we demonstrate that male mice homozygous for a targeted mutation of Mvh exhibit a reproductive deficiency. Male homozygotes produce no sperm in the testes, where premeiotic germ cells cease differentiation by the zygotene stage and undergo apoptotic death. In addition, the proliferation of PGCs that colonize homozygous male gonads is significantly hampered, and OCT-3/4 expression appears to be reduced. These results indicate that the loss ofMvh function causes a deficiency in the proliferation and differentiation of mouse male germ cells.

Organization of the yeast Zip1 protein within the central region of the synaptonemal complex

The yeast Zip1 protein is a component of the central region of the synaptonemal complex (SC). Zip1 is predicted to form an alpha-helical coiled coil, flanked by globular domains at the NH(2) and COOH termini. Immunogold labeling with domain-specific anti-Zip1 antibodies demonstrates that the NH(2)-terminal domain of Zip1 is located in the middle of the central region of the SC, whereas the COOH-terminal domain is embedded in the lateral elements of the complex. Previous studies have shown that overproduction of Zip1 results in the assembly of two types of aggregates, polycomplexes and networks, that are unassociated with chromatin. Our epitope mapping data indicate that the organization of Zip1 within polycomplexes is similar to that of the SC, whereas the organization of Zip1 within networks is fundamentally different. Zip1 protein purified from bacteria assembles into dimers in vitro, and electron microscopic analysis demonstrates that the two monomers within a dimer are arranged in parallel and in register. Together, these results suggest that two Zip1 dimers, lying head-to-head, span the width of the SC.

Temporal and spatial control of the Sycp1 gene transcription in the mouse meiosis: regulatory elements active in the male are not sufficient for expression in the female gonad

Transcription controls active at the initial stages of meiosis are clearly key elements in the regulation of germinal differentiation. Transcription of the Sycp1 gene (synaptonemal complex protein 1) starts as early as the leptotene and zygotene stages. Constructs with Sycp1 5' upstream sequences directed the expression of reporter genes to pachytene spermatocytes in transgenic mice. A short fragment encompassing the transcription start (n.t. -54 to +102) was sufficient for stage-specific expression in the adult male and for temporal regulation during development. Upstream enhancer element(s) quantitatively regulating expression were localized in the region between -54 and -260. The gene is normally expressed both in the male and female gonads, but none of the promoter sequences active in the testis allowed the expression of reporter genes during meiosis in the ovary.

Germ cell-specific enhancer activity of a repeated element in a variable region of the mouse genome

We recently described a complex genetic structure on mouse chromosome 8, a region of the murine genome in which genetic rearrangements frequently occur. A large repeated element specific to this chromosome was found to overlap with one of the cadherin genes (Cad11). An additional degree of complexity became apparent with the identification, in a subset of laboratory strains of mice, of a retrogene integrated into one of the repeated units. Designated Sycp1-ps2, it originated from the early meiotic gene encoding Synaptonemal Complex Protein 1. We now report that, among wild Mus species in which the retrogene is not present, this region of Chr 8 shows a high degree of variability. Sequence analysis showed that integration of Sycp1-ps2 created a 5' transcription initiator element. Transcription of the pseudogene in the testis was directly demonstrated. A germ cell-specific enhancer activity was localized within a 1117 bp region of the repeat, which was sufficient to direct the expression of reporter genes in transgenic mice to late meiotic and post-meiotic spermatogenic cells.

The synaptonemal complex protein SCP3 can form multistranded, cross-striated fibers in vivo

The synaptonemal complex protein SCP3 is part of the lateral element of the synaptonemal complex, a meiosis-specific protein structure essential for synapsis of homologous chromosomes. We have investigated the fiber-forming properties of SCP3 to elucidate its role in the synaptonemal complex. By synthesis of SCP3 in cultured somatic cells, it has been shown that SCP3 can self-assemble into thick fibers and that this process requires the COOH-terminal coiled coil domain of SCP3, as well as the NH2-terminal nonhelical domain. We have further analyzed the thick SCP3 fibers by transmission electron microscopy and immunoelectron microscopy. We found that the fibers display a transversal striation with a periodicity of approximately 20 nm and consist of a large number of closely associated, thin fibers, 5-10 nm in diameter. These features suggest that the SCP3 fibers are structurally related to intermediate filaments. It is known that in some species the lateral elements of the synaptonemal complex show a highly ordered striated structure resembling that of the SCP3 fibers. We propose that SCP3 fibers constitute the core of the lateral elements of the synaptonemal complex and function as a molecular framework to which other proteins attach, regulating DNA binding to the chromatid axis, sister chromatid cohesion, synapsis, and recombination.

Meiotic chromosome morphology and behavior in zip1 mutants of Saccharomyces cerevisiae

The yeast Zip1 protein (Zip1p) is a component of the central region of the synaptonemal complex (SC). Zip1p is predicted to form a dimer consisting of a coiled-coil domain flanked by globular domains. To analyze the organization of Zip1p within the SC, in-frame deletions of ZIP1 were constructed and analyzed. The results demonstrate that the C terminus but not the N terminus of Zip1p is required for its localization to chromosomes. Deletions in the carboxy half of the predicted coiled-coil region cause decreases in the width of the SC. Based on these results, a model for the organization of Zip1p within the SC is proposed. zip1 deletion mutations were also examined for their effects on sporulation, spore viability, crossing over, and crossover interference. The results demonstrate that the extent of synapsis is positively correlated with the levels of spore viability, crossing over, and crossover interference. In contrast, the role of Zip1p in synapsis is separable from its role in meiotic cell cycle progression. zip1 mutants display interval-specific effects on crossing over.

Identification of a meiosis-specific protein as a member of the class of cancer/testis antigens

Little is known about the function of human cancer/testis antigens (CTAs), such as MAGE, BAGE, GAGE, HOM-MEL-40, and NY-ESO-1, the expression of which is restricted to human malignancies and testis. When screening a cDNA expression library enriched for testis-specific representative long transcripts for reactivity with high-titered IgG antibodies from the serum of a patient with renal cell carcinoma, one repeatedly detected antigen, designated HOM-TES-14, turned out to be encoded by the synaptonemal complex protein 1 (SCP-1) gene. SCP-1 is known to be selectively expressed during the meiotic prophase of spermatocytes and is involved in the pairing of homologous chromosomes, an essential step for the generation of haploid cells in meiosis I. Investigation of a broad spectrum of normal and malignant tissues revealed expression of SCP-1 transcripts and antigen selectively in a variety of neoplastic tissues and tumor cell lines. Immunofluorescence microscopy analysis with specific antiserum showed a cell cycle phase-independent nuclear expression of SCP-1 protein in cancer cells. SCP-1 differs from other members of the class of CTA by its localization on chromosome 1 and its frequent expression in malignant gliomas, breast, renal cell, and ovarian cancer. The aberrant expression of SCP-1 in tumors might contribute to their genomic instability and suggests that the functional role of other CTA might also relate to meiosis.

Mapping of eight testis-specific genes to mouse chromosomes

We previously identified eight testis-specific genes using antibodies raised against testicular germ cells. They are expressed during spermatogenesis and are presumed to be involved in testicular germ cell differentiation and sperm formation. We have mapped the genomic loci for these testis-specific genes using restriction fragment length variants in interspecific backcross mice. The calmegin gene (Clgn) was mapped to Chr 8. The synaptonemal complex protein gene 1 (Sycp1) probe hybridized with two sequences on different chromosomes; Sycp1-rs2 was mapped to Chr 3, whereas Sycp1-rs3 was mapped to Chr 7. The relaxin-like factor gene (Rlnl) was mapped to Chr 8, and collapsin response mediator protein 1 (Crmp1) was mapped to Chr 5. Three novel genes encoding testis-specific proteins A2 (Tsga2), A8 (Tsga8), and A12 (Tsga12) were mapped to chromosomes 3, X, and 10, respectively.

Protein-protein interactions in the synaptonemal complex

In mammalian systems, an approximately M(r) 30,000 Cor1 protein has been identified as a major component of the meiotic prophase chromosome cores, and a M(r) 125,000 Syn1 protein is present between homologue cores where they are synapsed and form the synaptonemal complex (SC). Immunolocalization of these proteins during meiosis suggests possible homo- and heterotypic interactions between the two as well as possible interactions with yet unrecognized proteins. We used the two-hybrid system in the yeast Saccharomyces cerevisiae to detect possible protein-protein associations. Segments of hamsters Cor1 and Syn1 proteins were tested in various combinations for homo- and heterotypic interactions. In the cause of Cor1, homotypic interactions involve regions capable of coiled-coil formation, observation confirmed by in vitro affinity coprecipitation experiments. The two-hybrid assay detects no interaction of Cor1 protein with central and C-terminal fragments of Syn1 protein and no homotypic interactions involving these fragments of Syn1. Hamster Cor1 and Syn1 proteins both associate with the human ubiquitin-conjugation enzyme Hsubc9 as well as with the hamster Ubc9 homologue. The interactions between SC proteins and the Ubc9 protein may be significant for SC disassembly, which coincides with the repulsion of homologs by late prophase I, and also for the termination of sister centromere cohesiveness at anaphase II.