SPIN, a substrate in the MAP kinase pathway in mouse oocytes

Spin1z induces the male pathway in the chicken by down-regulating Tcf4

SPINDLIN1-Z (SPIN1Z), a member of the Spin/Ssty(Y-linked spermiogenesis specific transcript) protein family, participates in the early embryonic development process. Our previous RNA-seq analysis indicates that the level of Spin1z was abundantly expressed in male embryonic stem cells (ESCs) and primitive germ cells (PGCs), we speculate that Spin1z may play an important role in chicken male differentiation. Therefore, the loss- and gain-of-function experiments provide solid evidence that Spin1z is both necessary and sufficient to initiate male development in chicken. Furthermore, chromatin immunoprecipitation (ChIP) assay and the dual-luciferase assay was performed to further confirm that Spin1z contributed to chicken male differentiation by inhibiting the Tcf4 transcription. Our findings provide a novel insight into the molecular mechanism for chicken male differentiation.

Cell cycle-related genes p57kip2, Cdk5 and Spin in the pathogenesis of neural tube defects

In the field of developmental neurobiology, accurate and ordered regulation of the cell cycle and apoptosis are crucial factors contributing to the normal formation of the neural tube. Preliminary studies identified several genes involved in the development of neural tube defects. In this study, we established a model of developmental neural tube defects by administration of retinoic acid to pregnant rats. Gene chip hybridization analysis showed that genes related to the cell cycle and apoptosis, signal transduction, transcription and translation regulation, energy and metabolism, heat shock, and matrix and cytoskeletal proteins were all involved in the formation of developmental neural tube defects. Among these, cell cycle-related genes were predominant. Retinoic acid ment caused differential expression of three cell cycle-related genes p57kip2, Cdk5 and Spin, the expression levels of which were downregulated by retinoic acid and upregulated during normal neural tube formation. The results of this study indicate that cell cycle-related genes play an important role in the formation of neural tube defects. P57kip2, Cdk5 and Spin may be critical genes in the pathogenesis of neural tube defects.

Dynamic distribution of spindlin in nucleoli, nucleoplasm and spindle from primary oocytes to mature eggs and its critical function for oocyte-to-embryo transition in gibel carp

Spindlin (Spin) was thought as a maternal-effect factor associated with meiotic spindle. Its role for the oocyte-to-embryo transition was suggested in mouse, but its direct evidence for the function had been not obtained in other vertebrates. In this study, we used the CagSpin-specific antibody to investigate CagSpin expression pattern and distribution during oogenesis of gibel carp (Carassius auratus gibelio). First, the oocyte-specific expression pattern and dynamic distribution was revealed in nucleoli, nucleoplasm, and spindle from primary oocytes to mature eggs by immunofluorescence localization. In primary oocytes and growth stage oocytes, CagSpin accumulates in nucleoli in increasing numbers along with the oocyte growth, and its disassembly occurs in vitellogenic oocytes, which implicates that CagSpin may be a major component of a large number of nucleoli in fish growth oocytes. Then, co-localization of CagSpin and β-tubulin was revealed in meiotic spindle of mature egg, indicating that CagSpin is one spindle-associated factor. Moreover, microinjection of CagSpin-specific antibody into the fertilized eggs blocked the first cleavage, and found that the CagSpin depletion resulted in spindle assembly disturbance. Thereby, our study provided the first direct evidence for the critical oocyte-to-embryo transition function of Spin in vertebrates, and confirmed that Spin is one important maternal-effect factor that participates in oocyte growth, oocyte maturation, and oocyte-to-embryo transition.

Use of proteomics to identify highly abundant maternal factors that drive the egg-to-embryo transition

As IVF becomes an increasingly popular method for human reproduction, it is more critical than ever to understand the unique molecular composition of the mammalian oocyte. DNA microarray studies have successfully provided valuable information regarding the identity and dynamics of factors at the transcriptional level. However, the oocyte transcribes and stores a large amount of material that plays no obvious role in oogenesis, but instead is required to regulate embryogenesis. Therefore, an accurate picture of the functional state of the oocyte requires both transcriptional profiling and proteomics. Here, we summarize our previous studies of the oocyte proteome, and present new panels of oocyte proteins that we recently identified in screens of metaphase II-arrested mouse oocytes. Importantly, our studies indicate that several abundant oocyte proteins are not, as one might predict, ubiquitous housekeeping proteins, but instead are unique to the oocyte. Furthermore, mouse studies indicate that a number of these factors arise from maternal effect genes (MEGs). One of the identified MEG proteins, peptidylarginine deiminase 6, localizes to and is required for the formation of a poorly characterized, highly abundant cytoplasmic structure: the oocyte cytoplasmic lattices. Additionally, a number of other MEG-derived abundant proteins identified in our proteomic screens have been found by others to localize to another unique oocyte feature: the subcortical maternal complex. Based on these observations, we put forth the hypothesis that the mammalian oocyte contains several unique storage structures, which we have named maternal effect structures, that facilitate the oocyte-to-embryo transition.

Gene expression during the oocyte-to-embryo transition in mammals

The seminal question in modern developmental biology is the origins of new life arising from the unification of sperm and egg. The roots of this question begin from 19th to 20th century embryologists studying fertilization and embryogenesis. Although the revolution of molecular biology has yielded significant insight into the complexity of this process, the overall orchestration of genes, molecules, and cells is still not fully formed. Early mammalian development, specifically the oocyte-to-embryo transition, is essentially under "maternal command" from factors deposited in the cytoplasm during oocyte growth, independent of de novo transcription from the nascent embryo. Many of the advances in understanding this developmental period occurred in tandem with application of new methods and techniques from molecular biology, from protein electrophoresis to sequencing and assemblies of whole genomes. From this bed of knowledge, it appears that precise control of mRNA translation is a key regulator coordinating the molecular and cellular events occurring during oocyte-to-embryo transition. Notably, oocyte transcriptomes share, yet retain some uniqueness, common genetic motifs among all chordates. The common genetic motifs typically define fundamental processes critical for cellular maintenance, whereas the unique genetic features may be a source of variation and a substrate for sexual selection, genetic drift, or gene flow. One purpose for this complex interplay among genes, proteins, and cells may allow for evolution to transform and act upon the underlying processes, at molecular, structural and organismal levels, to increase diversity, which is the ultimate goal of sexual reproduction.

Characterization of Spindlin1 isoform2 in mouse testis

AIM

To investigate the expression of Spindlin 1 (Spin 1) isoform2 and assess its function in mouse testis.

METHODS

First, reverse-transcription polymerase chain reaction (RT-PCR) was used to determine whether Spin1 isoform2 is present in mouse testis. Then the expression patterns of the isoform between newborn and adult mice testes were compared by immunoblot analysis. Finally, the diversity of its localization in mice testes at different ages (days 0, 7, 14, 21, 28 and 60) was observed by immunohistochemistry. The localization of the protein in mouse sperm was also investigated by immunofluorescence.

RESULTS

The RT-PCR results show that Spin1 isoform2 is present in mouse testis. As shown by immunoblot analysis, the isoform was more highly expressed in adult testes compared with newborn testes. Interestingly, Spin1 isoform2 did not show up in the cytoplasm of primary spermatocytes until day 14. Also, the protein exists at the tail of the mouse sperm.

CONCLUSION

Spin1 isoform2 is a protein expressed highly in adult testis, which might be involved in spermatogenesis and could be necessary for normal sperm motility.

Overexpression of spindlin1 induces metaphase arrest and chromosomal instability

Spin/Ssty gene family is high conserved and very abundant transcript involved in gametogenesis, which was repeatedly detected in early embryo. Nevertheless, the biologic roles of the members are still largely unknown. Previously we have identified human gene spindlin1 as a homologue of the family from ovarian cancer cells, and reported that stable overexpression of spindlin1 could transform NIH3T3 cells and induce tumorigenesis in nude mouse. Here, we showed that spindlin1, as a nuclear protein, was relocated during mitosis. A fraction of spindlin1 proteins was dynamic distributed along mitotic spindle tubulin and enriched at midzone following anaphase entering. We also showed that transient overexpression of spindlin1 induced cell cycle delay in metaphase, caused mitotic spindle defects, and resulted in chromosome instability, micronucleus and multinuclear giant cells formation. Moreover, time-lapse microscopy revealed that these cells arrested at metaphase for more than 3 h with chromosome nondisjunction or missegregation. Furthermore, Mad2 up-regulation in these cells suggested that overexpression of spindlin1 may affect the bipolar spindle correctly attachment to chromosomes and activate spindle checkpoint. Taken together, these data demonstrated that excess spindlin1 protein may be detrimental for spindle microtubule organization, chromosomal stability and can potentially contribute to the development of cancer.

Transmission of Y chromosomes from XY female mice was made possible by the replacement of cytoplasm during oocyte maturation

The B6.Y(TIR) sex-reversed female mouse is anatomically normal at young ages but fails to produce offspring. We have previously shown that its oocytes go through the meiotic cell cycle up to the second metaphase; however, the meiotic spindle is not properly organized, the second meiotic division goes awry after activation or fertilization, and none of the oocytes initiate embryonic development. In the present study, we transferred the nuclei of GV-stage oocytes from XY females into the enucleated GV-stage oocytes from (B6.DBA)F1.XX females. The resultant reconstructed oocytes properly assembled second meiotic spindles after in vitro maturation and produced healthy offspring after in vitro fertilization. Some male pups inherited maternal Y chromosomes. We conclude that the cytoplasm of the XY oocyte is insufficient to support spindle formation at the second metaphase whereas its replacement with the cytoplasmic material from an XX oocyte allows normal development.

Oocyte quality and maternal control of development

The oocyte is a unique and highly specialized cell responsible for creating, activating, and controlling the embryonic genome, as well as supporting basic processes such as cellular homeostasis, metabolism, and cell cycle progression in the early embryo. During oogenesis, the oocyte accumulates a myriad of factors to execute these processes. Oogenesis is critically dependent upon correct oocyte-follicle cell interactions. Disruptions in oogenesis through environmental factors and changes in maternal health and physiology can compromise oocyte quality, leading to arrested development, reduced fertility, and epigenetic defects that affect long-term health of the offspring. Our expanding understanding of the molecular determinants of oocyte quality and how these determinants can be disrupted has revealed exciting new insights into the role of oocyte functions in development and evolution.

Structure of Human Spindlin1

Spindlin1, a meiotic spindle-binding protein that is highly expressed in ovarian cancer cells, was first identified as a gene involved in gametogenesis. It appeared to be a target for cell cycle-dependent phosphorylation and was demonstrated to disturb the cell cycle. Here we report the crystal structure of human spindlin1 to 2.2A of resolution, representing the first three-dimensional structure from the spin/ssty (Y-linked spermiogenesis-specific transcript) gene family. The refined structure, containing three repeats of five/four anti-parallel beta-strands, exhibits a novel arrangement of tandem Tudor-like domains. Two phosphate ions, chelated by Thr-95 and other residues, appear to stabilize the long loop between domains I and II, which might mediate the cell cycle regulation activity of spindlin1. Flow cytometry experiments indicate that cells expressing spindlin1 display a different cell cycle distribution in mitosis, whereas those expressing a T95A mutant, which had a great decrease in phosphorous content, have little effect on the cell cycle. We further identified associations of spindlin1 with nucleic acid to provide a biochemical basis for its cell cycle regulation and other functions.

Analysis of polysomal mRNA populations of mouse oocytes and zygotes: dynamic changes in maternal mRNA utilization and function

Transcriptional activation in mammalian embryos occurs in a stepwise manner. In mice, it begins at the late one-cell stage, followed by a minor wave of activation at the early two-cell stage, and then the major genome activation event (MGA) at the late two-cell stage. Cellular homeostasis, metabolism, cell cycle, and developmental events are orchestrated before MGA by time-dependent changes in the array of maternal transcripts being translated. Many elegant studies have documented the importance of maternal mRNA (MmRNA) and its correct recruitment for development. Many other studies have illuminated some of the molecular mechanisms regulating MmRNA utilization. However, neither the complete array of recruited mRNAs nor the regulatory mechanisms responsible for temporally different patterns of recruitment have been well characterized. We present a comprehensive analysis of changes in the maternal component of the zygotic polysomal mRNA population during the transition from oocyte to late one-cell stage embryo. We observe global transitions in the functional classes of translated MmRNAs and apparent changes in the underlying cis-regulatory mechanisms.

Cross-species hybridizations on a multi-species cDNA microarray to identify evolutionarily conserved genes expressed in oocytes

BACKGROUND

Comparative genomic analysis using cDNA microarray is a new approach and a useful tool to identify important genetic sequences or genes that are conserved throughout evolution. Identification of these conserved sequences will help elucidate important molecular mechanisms or pathways common to many species. For example, the stockpiled transcripts in the oocyte necessary for successful fertilization and early embryonic development still remain relatively unknown. The objective of this study was to identify genes expressed in oocytes and conserved in three evolutionarily distant species.

RESULTS

In this study we report the construction of a multi-species cDNA microarray containing 3,456 transcripts from three distinct oocyte-libraries from bovine, mouse and Xenopus laevis. Following the cross-species hybridizations, data analysis revealed that 1,541 positive hybridization signals were generated by oocytes of all three species, and 268 of these are preferentially expressed in the oocyte. Data reproducibility analyses comparing same-species to cross-species hybridization indicates that cross-species hybridizations are highly reproducible, thus increasing the confidence level in their specificity. A validation by RT-PCR using gene- and species-specific primers confirmed that cross-species hybridization allows the production of specific and reliable data. Finally, a second validation step through gene-specific microarray hybridizations further supported the validity of our cross-species microarray results. Results from these cross-species hybridizations on our multi-species cDNA microarray revealed that SMFN (Small fragment nuclease), Spin (Spindlin), and PRMT1 (Protein arginine methyltransferase 1) are transcripts present in oocytes and conserved in three evolutionarily distant species.

CONCLUSION

Cross-species hybridization using a multi-species cDNA microarray is a powerful tool for the discovery of genes involved in evolutionarily conserved molecular mechanisms. The present study identified conserved genes in the oocytes of three distant species that will help understand the unique role of maternal transcripts in early embryonic development.

Spindlin1, a novel nuclear protein with a role in the transformation of NIH3T3 cells

spindlin1, a novel human gene recently isolated by our laboratory, is highly homologous to mouse spindlin gene. In this study, we cloned cDNA full-length of this novel gene and send it to GenBank database as spindlin1 (Homo sapiens spindlin1) with Accession No. AF317228. In order to investigate the function of spindlin1, we studied further the subcellular localization of Spindlin1 protein and the effects of spindlin1 overexpression in NIH3T3 cells. The results showed that the fusion protein pEGFP-N1-spindlin1 was located in the nucleus and the C-terminal is correlated with nuclear localization of Spindlin1 protein. NIH3T3 cells which could stably express spindlin1 as a result of RT-PCR analysis compared with the control cells displayed a complete morphological change; made cell growth faster; and increased the percentage of cells in G2/M and S phase. Furthermore, overexpressed spindlin1 cells formed colonies in soft agar in vitro and formed tumors in nude mice. Our findings provide direct evidence that spindlin1 gene may contribute to tumorigenesis.

Eomesodermin is expressed in mouse oocytes and pre-implantation embryos

T-box genes are a highly conserved family of genes encoding transcription factors, which share a conserved DNA binding domain (the T-box). Appropriate temporal and spatial expression of this gene family is critical for gastrulation and organogenesis in a number of species. The T-box containing gene Eomesodermin was first identified in Xenopus, where it plays a critical role in mesoderm formation. In situ analyses in mice have described the expression patterns of the mouse ortholog of this gene mEomesodermin (mEomes) at the time of implantation and during fetal development. Additional studies involving the disruption of the mEomes gene, have demonstrated an additional role for mEomes in trophoblast formation. However, these analyses did not address the possibility that maternally encoded or pre-blastocyst zygotic transcription of mEomes may also contribute to embryonic development. We show here that mEomes mRNA is present prior to blastocyst formation, and that the protein product of mEomes is associated with nuclear DNA during oocyte development and persistently localizes within all nuclei of the preimplantation embryo until the early blastocyst stage. mEomes protein is associated with the meiotic spindle in the unfertilized egg and with the mitotic spindle at each cell division. Our results are consistent with mEomesodermin having a role in early preimplantation development and inner cell mass formation in addition to its function in the trophoblast lineage.

Identification of three duplicated Spin genes in medaka (Oryzias latipes)

Gene and genomic duplications are very important and frequent events in fish evolution, and the divergence of duplicated genes in sequences and functions is a focus of research on gene evolution. Here, we report the identification and characterization of three duplicated Spindlin (Spin) genes from medaka (Oryzias latipes): OlSpinA, OlSpinB, and OlSpinC. Molecular cloning, genomic DNA Blast analysis and phylogenetic relationship analysis demonstrated that the three duplicated OlSpin genes should belong to gene duplication. Furthermore, Western blot analysis revealed significant expression differences of the three OlSpins among different tissues and during embryogenesis in medaka, and suggested that sequence and functional divergence might have occurred in evolution among them.

Identification of a Spindlin homolog in gibel carp (Carassius auratus gibelio)

Spindlin has been suggested to play an important role during the transition from oocyte maturation to embryo development in mouse, but its homolog similar to the mouse Spindlin in molecular and expression characterization has not been identified up to now in other vertebrates. In this study, a full length of cDNA sequence is cloned and sequenced from the gibel carp (Carassius auratus gibelio). It contains 1240 nucleotides with an open reading frame of 771 nt encoding 257 amino acids. Based on its amino acid sequence alignment and comparison analysis with the known Spin family proteins, the newly cloned Spin is named Carassius auratus gibelio Spindlin (CagSpin). Its product could be detected from mature eggs to blastula embryos, but its content decreased from the two-cell stage, and could not be detected after the gastrula stage. It suggests that the CagSpin should be a maternal protein that is expressed during oocyte maturation, and plays a crucial role in early cleavage of embryogenesis. CagSpin is the first homolog similar to mouse spindlin identified in fish, and also in other vertebrates. GST pull-down assay reveals the first biochemical evidence for the association of CagSpin and beta-tubulin, the microtubule component. Therefore, CagSpin may play important functions by interacting with beta-tubulin and other spindle proteins during oocyte maturation and egg fertilization.

A new deletion of the mouse Y chromosome long arm associated with the loss of Ssty expression, abnormal sperm development and sterility

The mouse Y chromosome carries 10 distinct genes or gene families that have open reading frames suggestive of retained functionality; it has been assumed that many of these function in spermatogenesis. However, we have recently shown that only two Y genes, the testis determinant Sry and the translation initiation factor Eif2s3y, are essential for spermatogenesis to proceed to the round spermatid stage. Thus, any further substantive mouse Y-gene functions in spermatogenesis are likely to be during sperm differentiation. The complex Ssty gene family present on the mouse Y long arm (Yq) has been implicated in sperm development, with partial Yq deletions that reduce Ssty expression resulting in impaired fertilization efficiency. Here we report the identification of a more extensive Yq deletion that abolishes Ssty expression and results in severe sperm defects and sterility. This result establishes that genetic information (Ssty?) essential for normal sperm differentiation and function is present on mouse Yq.

A protein encoded by a member of the multicopy Ssty gene family located on the long arm of the mouse Y chromosome is expressed during sperm development

Multicopy Y-chromosomal genes in human and mouse have been postulated to play a role in spermatogenesis. The mouse Y long arm (Yq) carries hundreds of supposedly intronless copies of Ssty, for which no protein has hitherto been identified; mice lacking Yq are sterile with grossly abnormal sperm. We have now identified an Ssty-encoded protein (Ssty1) that is expressed in spermatids. The protein is absent from spermatids of mice that lack Yq, but is not reduced in mice with a two-thirds reduction of Ssty copies, implying that most do not produce this protein. Furthermore, no protein was produced by a strongly transcribed intronless Ssty transgene, raising doubts as to the protein-encoding potential of these intronless genes. We have now identified an intron-containing copy that is also present in multiple copies on Yq. One or more intron-containing copies are retained in the Ssty-deficient mice and may be the source of the Ssty1 protein.

Transgenic RNAi-mediated reduction of MSY2 in mouse oocytes results in reduced fertility

MSY2 is implicated in regulating the stability and translation of maternal mRNAs during mouse oogenesis. We report here that by driving the expression of a transgene encoding an Msy2 hairpin dsRNA in growing oocytes using the oocyte-specific Zp3 promoter, the amount of MSY2 protein was reduced by at least 60% in fully grown oocytes. The decrease appeared specific because no decrease was observed in either non-targeted mRNAs or proteins. Fertility of transgenic females was severely reduced. Although transgenic eggs could be inseminated, the eggs did not exhibit the normal series of oscillations in intracellular Ca2+, resume meiosis, undergo cortical granule exocytosis, or ZP2 cleavage to ZP2f. Transgenic oocytes also displayed a higher incidence of both the non-surrounded nucleolus chromatin morphology, and abnormal meiotic spindle formation was observed following oocyte maturation. Transgenic oocytes contained less total mRNA (approximately 75-80% that of non-transgenic oocytes) and displayed a reduced level of protein synthesis. Moreover, several of the maturation-associated changes in protein synthesis failed to occur in the transgenic oocytes. These results support a role for MSY2 in stabilizing maternal mRNAs in growing oocytes, a process essential to generate meiotically and developmentally competent oocytes.

A New Deletion of the Mouse Y Chromosome Long Arm Associated With the Loss of Ssty Expression, Abnormal Sperm Development and Sterility

The mouse Y chromosome carries 10 distinct genes or gene families that have open reading frames suggestive of retained functionality; it has been assumed that many of these function in spermatogenesis. However, we have recently shown that only two Y genes, the testis determinant Sry and the translation initiation factor Eif2s3y, are essential for spermatogenesis to proceed to the round spermatid stage. Thus, any further substantive mouse Y-gene functions in spermatogenesis are likely to be during sperm differentiation. The complex Ssty gene family present on the mouse Y long arm (Yq) has been implicated in sperm development, with partial Yq deletions that reduce Ssty expression resulting in impaired fertilization efficiency. Here we report the identification of a more extensive Yq deletion that abolishes Ssty expression and results in severe sperm defects and sterility. This result establishes that genetic information (Ssty?) essential for normal sperm differentiation and function is present on mouse Yq.

Functional cloning of SPIN-2, a nuclear anti-apoptotic protein with roles in cell cycle progression

The balance between hematopoietic cell viability and apoptosis is regulated by exogenous growth factors, however, the molecular mechanisms by which these trophic factors exert their effects remain obscure. A functional retroviral cDNA library-based screen was employed to identify genes that prevent growth factor withdrawal-mediated apoptosis in the myeloid progenitor cell 32Dcl3. This approach identified three classes of genes: those with known roles in apoptosis (bcl-X(L) and ornithine decarboxylase); genes previously identified but not linked directly to apoptotic signaling (O-linked N-acetylglucosamine transferase); and a previously uncharacterized gene we termed SPIN-2. In 32Dcl3 cells, expression of exogenous SPIN-2 provides 25% protection from apoptosis following growth factor withdrawal compared to controls which show approximately 1-2% survival. SPIN-2 overexpression slows cell growth rates and increases the percentage of cells in G(2)/M (32% vs control cells at 12%). Immunolocalization studies indicate that myc-epitope tagged SPIN-2 proteins, which retain their anti-apoptotic function, reside in the nucleus, whereas a C-terminal deletion mutant that loses its anti-apoptotic activity is located in the cytoplasm. These studies suggest that SPIN-2 is a novel nuclear protein that functions to regulate cell cycle progression and this activity is related to the inhibition of apoptosis following the removal of essential growth factors.

Association of MPF, MAPK, and nuclear progression dynamics during activation of young and aged bovine oocytes

We have previously shown that bovine oocytes parthenogenetically activated after 40 hours (hr) of in vitro maturation proceed through the cell cycle faster than those after 20 hr of maturation. In the present study, we used this model of different speed of nuclear progression to investigate the correlation of two hallmarks of nuclear events, exit of metaphase arrest and pronuclear formation, with dynamics of MPF and MAPK. Bovine oocytes were matured in vitro for 20 hr (young) or 40 hr (aged) and activated in 7% ethanol followed by incubation in cycloheximide for 0, 0.5, 1, 3, 5, or 7 hr. Activity of MPF and MAPK was lower in aged than young oocytes. The responses to oocyte activation by both the two kinases and nuclear progression were faster in aged than in young oocytes. The activity of MPF declined to undetectable levels (P < 0.05) as early as 0.5 hr after activation in aged oocytes, while this did not happen in young oocytes until 3 hr after activation. The inactivation of MAPK occurred approximately 2 hr earlier in aged oocytes (5 hr post-activation) than in young oocytes (7 hr post-activation). Furthermore, the decline in MPF activity preceded that of MAPK in both young and aged oocytes by about 2 hr. The decrease in activity of MPF and MAPK corresponded with the exit from meiosis and pronuclei formation regardless of the speed of nuclear progression. Despite dramatic changes in activity of MPF and MAPK, the levels of Cdc2 and Erk2 proteins were unchanged (P > 0.05) during the first 7 hr of activation. These observations suggest that inactivation of MPF and MAPK are pre-requisite for the release from metaphase arrest and formation of pronuclei in bovine oocytes.

Chicken spindlin genes on W and Z chromosomes: transcriptional expression of both genes and dynamic behavior of spindlin in interphase and mitotic cells

Contigs of genomic clones covering about 480 kb on the terminal region of the short arm of chicken W chromosome were obtained. By applying the exon trapping procedure on this whole region, a chicken homolog of spindlin gene, chSpin-W, was identified and subcloned. A counterpart gene, chSpin-Z, was found near the centromere on the long arm of Z chromosome. Although protein-coding regions of both genes are nearly identical, a part of the 3'-untranslated region is sufficiently different to distinguish the transcript of chSpin-W. Both chSpin-W and chSpin-Z are transcribed in early embryos. chSpin-Z is transcribed in various tissues of adult chickens, while chSpin-W is transcribed most prominently in ovarian granulosa and thecal cells. When female chicken embryonic fibroblasts were transfected with a cDNA construct for red fluorescent protein or green fluorescent protein-fused spindlin or FLAG-tagged spindlin, the expressed spindlin was co-localized with SUMO-1 in nuclear dots, ND10, in interphase cells, while the expressed spindlin was localized on entire chromosomes during mitosis. The localization of spindlin in ND10 reappeared after mitosis in daughter cell nuclei. A C-terminal region of spindlin was suggested to be required for the localization of spindlin to ND10.

The Spin/Ssty repeat: a new motif identified in proteins involved in vertebrate development from gamete to embryo

BACKGROUND

The homologous genes Spin (spindlin) and Ssty were first identified as genes involved in gametogenesis and seem to occur in multiple copies in vertebrate genomes. The mouse spindlin (Spin) protein was reported to interact with the spindle apparatus during oogenesis and to be a target for cell-cycle-dependent phosphorylation. The transcript of the mouse Ssty gene is specific to sperm cells. In the chicken, spindlin was found to co-localize with SUMO-1 to nuclear dots during interphase in fibroblasts, but to co-localize with chromosomes during mitosis. Thus, Spin/Ssty genes might be important in the transition from sperm cells and oocytes to the early embryo, as well as in mitosis.

RESULTS

Here we report the discovery of a new protein motif of around 50 amino acids in length, the Spin/Ssty repeat, in proteins of the Spin/Ssty (spindlin) family. We found that in one member of this family, the human SPIN gene, each repeat resides in its own exon, supporting our view that Spin/Ssty repeats are independent functional units. On the basis of different secondary-structure prediction methods, we propose a four-stranded beta-structure for the Spin/Ssty repeat.

CONCLUSIONS

The discovery of the Spin/Ssty repeat might contribute to the further elucidation of the structure and function of spindlin-family proteins. We predict that the tertiary structure of spindlin-like proteins is composed of three modules of Spin/Ssty repeats.

Expression of genes involved in mammalian meiosis during the transition from egg to embryo

The ooplasm of higher eukaryotes provides substances necessary for completing the last stages of meiosis and initiating the first mitotic division. These processes are firmly attuned to other events in the egg and newly formed embryo, such as switching from the use of maternal transcripts to the onset of zygotic transcription. In mammals little is known about the molecular mechanisms guiding this transition, largely due to the lack of information about genes expressed in the egg and early embryos. Studies of yeast mitosis have contributed much of what is known about the vertebrate cell cycle, and recent reports indicate that homologs of yeast DNA repair genes also function during mammalian gametogenesis. To examine whether this conservation can be expanded to include genes operative in oocyte meiosis, we performed a computer-based search for homologs of yeast genes that are induced during sporulation in C. elegans, Drosophila, and mammals. Results from this study suggest that yeast and higher eukaryotes share genes that coordinate the overall process of meiosis. However intriguing differences exist, reflecting the distinctive mechanisms governing the progression of meiosis in each organism. ESTs representing more than half of the mammalian homologs are present in mouse cDNA libraries that contains genes controlling the meiosis/mitosis transition. About 50% of these genes contain potential cis-elements for cytoplasmic polyadenylation in their 3'-UTR, suggesting the importance of controlled translation in the egg and zygote.

Timely translation during the mouse oocyte-to-embryo transition

In the mouse, completion of oocyte maturation and the initiation of preimplantation development occur during transcriptional silence and depend on the presence and translation of stored mRNAs transcribed in the growing oocyte. The Spin gene has three transcripts, each with an identical open reading frame and a different 3′ untranslated region (UTR). (Beta)-galactosidase-tagged reporter transcripts containing each of the different Spin 3′UTRs were injected into oocytes and zygotes and (beta)-galactosidase activity was monitored. Results from these experiments suggest that differential polyadenylation and translation occurs at two critical points in the oocyte-to-embryo transition - upon oocyte maturation and fertilization - and is dependent on sequences in the 3′UTR. The stability and mobility shifts of ten other maternal transcripts were monitored by reprobing a northern blot of oocytes and embryos collected at 12 hour intervals after fertilization. Some are more stable than others and the upward mobility shift associated with polyadenylation correlates with the presence of cytoplasmic polyadenylation elements (CPEs) within about 120 nucleotides of the nuclear polyadenylation signal. A survey of the 3′ UTRs of expressed sequence tag clusters from a mouse 2-cell stage cDNA library indicates that about one third contain CPEs. We suggest that differential transcript stability and a translational control program can supply the diversity of protein products necessary for oocyte maturation and the initiation of development.

Expression of Melk, a new protein kinase, during early mouse development

A new gene named maternal embryonic leucine zipper kinase, Melk, has been recently identified (Heyer et al. [1997] Mol. Reprod. Dev. 47:148-156). As a basis for further study of the function of the gene, we have examined the expression of Melk across a wide range of embryonic stages, from the ovulated egg and 2-cell embryo through the gastrulation and early organogenesis stages, by in situ hybridization and immunohistochemistry. Melk is expressed in a spatially and temporally specific pattern during mammalian embryogenesis. The strongest expression was detected during maturation of oocytes and preimplantation development. Given its expression pattern, Melk may play an important role during preimplantation embryonic development. Dev Dyn 1999;215:344-351.