Murine polo like kinase 1 gene is expressed in meiotic testicular germ cells and oocytes

Understanding the Underlying Molecular Mechanisms of Meiotic Arrest during In Vitro Spermatogenesis in Rat Prepubertal Testicular Tissue

In vitro spermatogenesis appears to be a promising approach to restore the fertility of childhood cancer survivors. The rat model has proven to be challenging, since germ cell maturation is arrested in organotypic cultures. Here, we report that, despite a meiotic entry, abnormal synaptonemal complexes were found in spermatocytes, and in vitro matured rat prepubertal testicular tissues displayed an immature phenotype. RNA-sequencing analyses highlighted up to 600 differentially expressed genes between in vitro and in vivo conditions, including genes involved in blood-testis barrier (BTB) formation and steroidogenesis. BTB integrity, the expression of two steroidogenic enzymes, and androgen receptors were indeed altered in vitro. Moreover, most of the top 10 predicted upstream regulators of deregulated genes were involved in inflammatory processes or immune cell recruitment. However, none of the three anti-inflammatory molecules tested in this study promoted meiotic progression. By analysing for the first time in vitro matured rat prepubertal testicular tissues at the molecular level, we uncovered the deregulation of several genes and revealed that defective BTB function, altered steroidogenic pathway, and probably inflammation, could be at the origin of meiotic arrest.

Modelling the Functions of Polo-Like Kinases in Mice and Their Applications as Cancer Targets with a Special Focus on Ovarian Cancer

Polo-like kinases (PLKs) belong to a five-membered family of highly conserved serine/threonine kinases (PLK1-5) that play differentiated and essential roles as key mitotic kinases and cell cycle regulators and with this in proliferation and cellular growth. Besides, evidence is accumulating for complex and vital non-mitotic functions of PLKs. Dysregulation of PLKs is widely associated with tumorigenesis and by this, PLKs have gained increasing significance as attractive targets in cancer with diagnostic, prognostic and therapeutic potential. PLK1 has proved to have strong clinical relevance as it was found to be over-expressed in different cancer types and linked to poor patient prognosis. Targeting the diverse functions of PLKs (tumor suppressor, oncogenic) are currently at the center of numerous investigations in particular with the inhibition of PLK1 and PLK4, respectively in multiple cancer trials. Functions of PLKs and the effects of their inhibition have been extensively studied in cancer cell culture models but information is rare on how these drugs affect benign tissues and organs. As a step further towards clinical application as cancer targets, mouse models therefore play a central role. Modelling PLK function in animal models, e.g., by gene disruption or by treatment with small molecule PLK inhibitors offers promising possibilities to unveil the biological significance of PLKs in cancer maintenance and progression and give important information on PLKs' applicability as cancer targets. In this review we aim at summarizing the approaches of modelling PLK function in mice so far with a special glimpse on the significance of PLKs in ovarian cancer and of orthotopic cancer models used in this fatal malignancy.

Novel targets identified by integrated proteomic and phosphoproteomic analysis in spermatogenesis of swamp buffalo (Bubalus bubalis)

To understand mechanisms of spermatogenesis, the proteome and the phosphoproteome in prepubertal and pubertal swamp buffalo (Bubalus bubalis) testes were analyzed using tandem mass tag (TMT) coupled with liquid chromatography-tandem mass spectrometry (LC–MS/MS). In prepubertal testes, 80 proteins were overexpressed, 148 proteins were underexpressed, and 139 and 142 protein sites had higher and lower phosphorylation, respectively, compared to the levels in pubertal testes. Several of these proteins were associated with reproductive processes such as sexual reproduction, spermatogenesis, fertilization, and spermatid development. In particular, outer dense fiber protein 1 (ODF1), protein maelstrom homolog (MAEL), actin-like protein 7B (ACTL7B), tyrosine-(Y)-phosphorylation regulated (CABYR), and tripartite motif containing 36 (TRIM36) were upregulated with age at both the proteome and phosphoproteome levels. Combining proteome and phosphoproteome analysis can be effectively applied to study the protein/phosphorylation patterns of buffalo testes. These data provide new regulatory candidates and evidence for a complex network in spermatogenesis in buffalo testes, and serve as an important resource for exploring the physiological mechanism of spermatogenesis in mammals.

Systematic analysis of the phosphoproteome and kinase-substrate networks in the mouse testis

Spermatogenesis is a complex process closely associated with the phosphorylation-orchestrated cell cycle. Elucidating the phosphorylation-based regulations should advance our understanding of the underlying molecular mechanisms. Here we present an integrative study of phosphorylation events in the testis. Large-scale phosphoproteome profiling in the adult mouse testis identified 17,829 phosphorylation sites in 3955 phosphoproteins. Although only approximately half of the phosphorylation sites enriched by IMAC were also captured by TiO2, both the phosphoprotein data sets identified by the two methods significantly enriched the functional annotation of spermatogenesis. Thus, the phosphoproteome profiled in this study is a highly useful snapshot of the phosphorylation events in spermatogenesis. To further understand phosphoregulation in the testis, the site-specific kinase-substrate relations were computationally predicted for reconstructing kinase-substrate phosphorylation networks. A core sub-kinase-substrate phosphorylation networks among the spermatogenesis-related proteins was retrieved and analyzed to explore the phosphoregulation during spermatogenesis. Moreover, network-based analyses demonstrated that a number of protein kinases such as MAPKs, CDK2, and CDC2 with statistically more site-specific kinase-substrate relations might have significantly higher activities and play an essential role in spermatogenesis, and the predictions were consistent with previous studies on the regulatory roles of these kinases. In particular, the analyses proposed that the activities of POLO-like kinases (PLKs) might be dramatically higher, while the prediction was experimentally validated by detecting and comparing the phosphorylation levels of pT210, an indicator of PLK1 activation, in testis and other tissues. Further experiments showed that the inhibition of POLO-like kinases decreases cell proliferation by inducing G2/M cell cycle arrest. Taken together, this systematic study provides a global landscape of phosphoregulation in the testis, and should prove to be of value in future studies of spermatogenesis.

TCTEX1D4 interactome in human testis: unraveling the function of dynein light chain in spermatozoa

Studies were designed to identify the TCTEX1D4 interactome in human testis, with the purpose of unraveling putative protein complexes essential to male reproduction and thus novel TCTEX1D4 functions. TCTEX1D4 is a dynein light chain that belongs to the DYNT1/TCTEX1 family. In spermatozoa, it appears to be important to sperm motility, intraflagellar transport, and acrosome reaction. To contribute to the knowledge on TCTEX1D4 function in testis and spermatozoa, a yeast two-hybrid assay was performed in testis, which allowed the identification of 40 novel TCTEX1D4 interactors. Curiously, another dynein light chain, TCTEX1D2, was identified and its existence demonstrated for the first time in human spermatozoa. Immunofluorescence studies proved that TCTEX1D2 is an intra-acrosomal protein also present in the midpiece, suggesting a role in cargo movement in human spermatozoa. Further, an in silico profile of TCTEX1D4 revealed that most TCTEX1D4 interacting proteins were not previously characterized and the ones described present a very broad nature. This reinforces TCTEX1D4 as a dynein light chain that is capable of interacting with a variety of functionally different proteins. These observations collectively contribute to a deeper molecular understanding of the human spermatozoa function.

Small-molecule inhibition of BRDT for male contraception

A pharmacologic approach to male contraception remains a longstanding challenge in medicine. Toward this objective, we explored the spermatogenic effects of a selective small-molecule inhibitor (JQ1) of the bromodomain and extraterminal (BET) subfamily of epigenetic reader proteins. Here, we report potent inhibition of the testis-specific member BRDT, which is essential for chromatin remodeling during spermatogenesis. Biochemical and crystallographic studies confirm that occupancy of the BRDT acetyl-lysine binding pocket by JQ1 prevents recognition of acetylated histone H4. Treatment of mice with JQ1 reduced seminiferous tubule area, testis size, and spermatozoa number and motility without affecting hormone levels. Although JQ1-treated males mate normally, inhibitory effects of JQ1 evident at the spermatocyte and round spermatid stages cause a complete and reversible contraceptive effect. These data establish a new contraceptive that can cross the blood:testis boundary and inhibit bromodomain activity during spermatogenesis, providing a lead compound targeting the male germ cell for contraception.

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Male hypogonadism and germ cell loss caused by a mutation in Polo-like kinase 4

The genetic etiologies of male infertility remain largely unknown. To identify genes potentially involved in spermatogenesis and male infertility, we performed genome-wide mutagenesis in mice with N-ethyl-N-nitrosourea and identified a line with dominant hypogonadism and patchy germ cell loss. Genomic mapping and DNA sequence analysis identified a novel heterozygous missense mutation in the kinase domain of Polo-like kinase 4 (Plk4), altering an isoleucine to asparagine at residue 242 (I242N). Genetic complementation studies using a gene trap line with disruption in the Plk4 locus confirmed that the putative Plk4 missense mutation was causative. Plk4 is known to be involved in centriole formation and cell cycle progression. However, a specific role in mammalian spermatogenesis has not been examined. PLK4 was highly expressed in the testes both pre- and postnatally. In the adult, PLK4 expression was first detected in stage VIII pachytene spermatocytes and was present through step 16 elongated spermatids. Because the homozygous Plk4(I242N/I242N) mutation was embryonic lethal, all analyses were performed using the heterozygous Plk4(+/I242N) mice. Testis size was reduced by 17%, and histology revealed discrete regions of germ cell loss, leaving only Sertoli cells in these defective tubules. Testis cord formation (embryonic day 13.5) was normal. Testis histology was also normal at postnatal day (P)1, but germ cell loss was detected at P10 and subsequent ages. We conclude that the I242N heterozygous mutation in PLK4 is causative for patchy germ cell loss beginning at P10, suggesting a role for PLK4 during the initiation of spermatogenesis.

Polo-like kinase Cdc5 drives exit from pachytene during budding yeast meiosis

In budding yeast, exit from the pachytene stage of meiosis requires the mid-meiosis transcription factor Ndt80, which promotes expression of approximately 200 genes. Ndt80 is required for meiotic function of polo-like kinase (PLK, Cdc5) and cyclin-dependent kinase (CDK), two cell cycle kinases previously implicated in pachytene exit. We show that ongoing CDK activity is dispensable for two events that accompany exit from pachytene: crossover formation and synaptonemal complex breakdown. In contrast, CDC5 expression in ndt80Delta mutants efficiently promotes both events. Thus, Cdc5 is the only member of the Ndt80 transcriptome required for this critical step in meiotic progression.

Genomic origin, processing and developmental expression of testicular outer dense fiber 2 (ODF2) transcripts and a novel nucleolar localization of ODF2 protein

Outer dense fibers are a major constituent of the sperm tail and outer dense fiber 2 (ODF2) protein is one of their major components. ODF2 shares partial homology with cenexin 1 and cenexin 2, regarded as centriolar proteins. We show that ODF2 and cenexin 2 transcripts are the product of differential splicing of a single gene, designated Cenexin/ODF2 and that cenexin 1 is an incomplete clone of ODF2. ODF2 terminates in exon 20b whereas in cenexin 2 this exon is spliced out and translation terminates in exon 24. We demonstrate a transcriptional switch during rat testicular development, from somatic-type to testis-type ODF2 and cenexin transcripts during the onset of meiosis. The switch is completed when spermiogenesis is established. ODF2 immunoreactive sites were visualized in the acroplaxome, along the sperm tail and the centrosome-derived sperm head-to-tail coupling apparatus. An unexpected finding was the presence of ODF2 antigenic sites, but not cenexin antigenic sites, in the dense fibrillar component of the nucleolus of Sertoli cells, spermatogonia and primary spermatocytes. The characterization of the genomic origin, processing and developmental expression of ODF2 transcript isoforms and their protein products can help reconcile differences in the literature on the role of ODF2 and cenexin in the centrosome. Furthermore, the finding of ODF2 in the dense fibrillar component of the nucleolus suggests that this protein, in addition to its presence in sperm outer dense fibers and centrosome, highlights and adds to the nucleolar function during spermatogenesis and early embryogenesis.

Molecular cloning of Plk1 and Nek2 and their expression in mature gonads of the teleost fish Nile tilapia (Oreochromis niloticus)

Polo-like kinase 1 (Plk1) and NIMA-related kinase 2 (Nek2) are serine/threonine kinases that are involved in the G2/M phase transition of the cell cycle in vertebrates. Although they also play critical roles in the regulation of spermatogenesis and oogenesis, their characterization in meiosis has not been elucidated fully, particularly in teleost fish. To investigate the evolutionary significance of serine/threonine kinases in the reproductive system of fish, we cloned the cDNAs of Plk1 and Nek2 from a Nile tilapia (Oreochromis niloticus) testicular cDNA library. Tilapia Plk1 encodes a protein of 582 amino acids that shares 75% homology with human Plk1, while tilapia Nek2 encodes a putative protein of 446 amino acids that shares 70% homology with human Nek2. Analyses of tissue distribution by RT-PCR and Southern blotting revealed that Plk1 and Nek2 are strongly expressed in the ovary and testis. Northern blot analysis revealed two Nek2 transcripts in the ovary and testis with different expression patterns, which indicates the presence of two structural variants for tilapia Nek2. Moreover, the localization of Plk1 and Nek2 mRNAs in tilapia gonads was determined by in situ hybridization analysis. In the ovary, Plk1 and Nek2 were expressed predominantly in oocytes. In the testis, on the other hand, Plk1 was expressed in primary spermatocytes, while Nek2 was generally expressed in primary and secondary spermatocytes. These results suggest that Plk1 and Nek2 are key factors in the progression of meiosis in fish.

Mechanisms and chemical induction of aneuploidy in rodent germ cells

The objective of this review is to suggest that the advances being made in our understanding of the molecular events surrounding chromosome segregation in non-mammalian and somatic cell models be considered when designing experiments for studying aneuploidy in mammalian germ cells. Accurate chromosome segregation requires the temporal control and unique interactions among a vast array of proteins and cellular organelles. Abnormal function and temporal disarray among these, and others to be identified, biochemical reactions and cellular organelles have the potential for predisposing cells to aneuploidy. Although numerous studies have demonstrated that certain chemicals (mainly those that alter microtubule function) can induce aneuploidy in mammalian germ cells, it seems relevant to point out that such data can be influenced by gender, meiotic stage, and time of cell-fixation post-treatment. Additionally, a consensus has not been reached regarding which of several germ cell aneuploidy assays most accurately reflects the human condition. More recent studies have shown that certain kinase, phosphatase, proteasome, and topoisomerase inhibitors can also induce aneuploidy in rodent germ cells. We suggest that molecular approaches be prudently incorporated into mammalian germ cell aneuploidy research in order to eventually understand the causes and mechanisms of human aneuploidy. Such an enormous undertaking would benefit from collaboration among scientists representing several disciplines.

Differential regulation of polo-like kinase 1, 2, 3, and 4 gene expression in mammalian cells and tissues

The four mammalian polo-like kinase (Plk) family members are critical regulators of cell cycle progression, mitosis, cytokinesis, and the DNA damage response. Research conducted to date has primarily investigated the expression patterns, structural features, substrates, and subcellular distribution of these important serine-threonine kinases. Here, we review the published data describing the regulation of Plk1, 2, 3, or 4 gene expression either during mammalian cell cycle progression or in tissue samples. These studies have demonstrated that the Plk family genes are differentially expressed following growth factor stimulation of quiescent fibroblasts. Furthermore, although Plk1 and Plk2 mRNA and protein levels are coordinately regulated during cell cycle progression, this is not the case for Plk3. In addition, the Plk1, 2 and 4 proteins have relatively short intracellular half-lives, but Plk3 is very stable. The Plk family genes are also differentially regulated in stressed cells; for example, when DNA-damaging agents are added to cycling cells, Plk1 expression decreases, but Plk2 and Plk3 expression increases. Finally, Plk1, 2, 3, and 4 are expressed to varying degrees in different human tissue types and it has been reported that Plk1 expression is increased and Plk3 expression is decreased in tumor specimens. These results indicate that the differential regulation of Plk family member gene expression is one cellular strategy for controlling Plk activity in mammalian cells.

A new role for the mitotic RAD21/SCC1 cohesin in meiotic chromosome cohesion and segregation in the mouse

The evolutionarily conserved cohesin complex is required for the establishment and maintenance of sister chromatid cohesion, in turn essential for proper chromosome segregation. RAD21/SCC1 is a regulatory subunit of the mitotic cohesin complex, as it links together all other subunits of the complex. The destruction of RAD21/SCC1 along chromosomal arms and later at centromeres results in the dissociation of the cohesin complex, facilitating chromosome segregation. Here, we report for the first time that mammalian RAD21/SCC1 associates with the axial/lateral elements of the synaptonemal complex along chromosome arms and on centromeres of mouse spermatocytes. Importantly, RAD21/SCC1 is lost from chromosome arms in late prophase I but persists on centromeres. The loss of centromeric RAD21/SCC1 coincides with the separation of sister chromatids at anaphase II. These findings support a role for mammalian RAD21/SCC1 in maintaining sister chromatid cohesion in meiosis.

Germ cell differentiation and synaptonemal complex formation are disrupted in CPEB knockout mice

CPEB is a sequence-specific RNA binding protein that regulates translation during vertebrate oocyte maturation. Adult female CPEB knockout mice contained vestigial ovaries that were devoid of oocytes; ovaries from mid-gestation embryos contained oocytes that were arrested at the pachytene stage. Male CPEB null mice also contained germ cells arrested at pachytene. The germ cells from the knockout mice harbored fragmented chromatin, suggesting a possible defect in homologous chromosome adhesion or synapsis. Two CPE-containing synaptonemal complex protein mRNAs, which interact with CPEB in vitro and in vivo, contained shortened poly(A) tails and mostly failed to sediment with polysomes in the null mice. Synaptonemal complexes were not detected in these animals. CPEB therefore controls germ cell differentiation by regulating the formation of the synaptonemal complex.

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.

Members of the ErbB receptor tyrosine kinases are involved in germ cell development in fetal mouse gonads

To isolate the genes involved in mouse primordial germ cell (PGC) development, we carried out subtraction cDNA cloning between PGC-derived embryonic germ (EG) cells and inner cell mass-derived embryonic stem cells. Among the genes preferentially expressed in EG cells, we found a gene encoding a receptor tyrosine kinase ErbB3. By in situ hybridization and immunohistochemical staining, the expression of ErbB3 as well as that of ErbB2, a coreceptor for ErbB3, was detected in PGCs in genital ridges at 12.5 dpc (days postcoitum). The expression was, however, downregulated at 14.5 dpc when the PGCs underwent growth cessation. Neuregulin-beta, a ligand for ErbB2 and ErbB3, was also expressed in genital ridges. In addition, a recombinant Neuregulin-beta enhanced the number of PGCs in 12.5-dpc embryos in culture. Taken together, these observations suggest that ErbB signaling controls the growth or survival of PGCs in genital ridges.

The transcriptional program of sporulation in budding yeast

Diploid cells of budding yeast produce haploid cells through the developmental program of sporulation, which consists of meiosis and spore morphogenesis. DNA microarrays containing nearly every yeast gene were used to assay changes in gene expression during sporulation. At least seven distinct temporal patterns of induction were observed. The transcription factor Ndt80 appeared to be important for induction of a large group of genes at the end of meiotic prophase. Consensus sequences known or proposed to be responsible for temporal regulation could be identified solely from analysis of sequences of coordinately expressed genes. The temporal expression pattern provided clues to potential functions of hundreds of previously uncharacterized genes, some of which have vertebrate homologs that may function during gametogenesis.

Molecular cloning and characterization of meichroacidin (male meiotic metaphase chromosome-associated acidic protein)

We have isolated a cDNA clone encoding a germ cell specific protein from an expression cDNA library prepared from the mouse testis, using testis-specific polyclonal antibodies. Sequence analysis of the cDNA revealed that the deduced amino acid sequence consisted of 284 residues, including a nominal repeat structure in the N-terminal region. Northern blot analysis revealed the presence of a transcript of 1.3 kb exclusively expressed in the testis and ovary, but at relatively low levels in the ovary. In contrast, no other tissues and organs expressed significant levels of the transcript. Expression of the mRNA in the testis was first detected on day 14 in postnatal development. Western blot analysis showed the presence of the protein with a molecular weight of approximately 40 kDa and an isoelectric point of 4.9. The protein was exclusively found in the testis and ovary, but in a far lesser amount in the ovary as was the case with the transcript. Immunohistochemical examination revealed that the protein was predominantly present in the cytoplasm in pachytene spermatocytes through to round spermatids. However, during the disappearance of the nuclear envelope at both the first and second meiotic divisions, the protein was localized around the metaphase chromosomes and spindles. Because of this, the name meichroacidin which stands for male meiotic metaphase chromosome-associated acidic protein is proposed for this antigen. The highly regulated stage-specific expression of meichroacidin and its specific association with the metaphase chromosomes and spindles suggest that the protein plays important roles in male meiosis.

A novel serine/threonine kinase gene, Gek1, is expressed in meiotic testicular germ cells and primordial germ cells

We have isolated a novel serine/ threonine kinase gene designated Gek1 from mouse primordial germ cell-derived embryonic germ cell. Gek1 is preferentially expressed in meiotic testicular germ cells and primordial germ cells. Gek1 mRNA is also detected in several other tissues, including hematopoietic organs in adult mice and central nervous system in embryos. The Gek1 cDNA encodes a protein with the consensus sequence of the catalytic domain of protein kinases in its N-terminal region. The deduced amino acid sequence of Gek1 in the kinase domain is related to those encoded by the Saccharomyces cerevisiae STE20, CDC15, and Drosophila melanogaster ninaC. The patterns of expression and the structural features of Gek1 suggest that the gene product is involved in signal transduction or nuclear division of germ cells and other proliferating cells. We also show that Gek1 locates on chromosome 11, near the wr locus, showing neuronal and reproductive defects.