A role for cyclin A1 in the activation of MPF and G2-M transition during meiosis of male germ cells in mice

Pluripotency factor Tex10 finetunes Wnt signaling for spermatogenesis and primordial germ cell development

Testis-specific transcript 10 (Tex10) is highly expressed in the testis, embryonic stem cells (ESCs), and primordial germ cells (PGCs). We previously generated a Tex10 knockout mouse model demonstrating its critical roles in ESC pluripotency and preimplantation development. Here, using conditional knockout mice and dTAG-degron ESCs, we show Tex10 is required for spermatogenesis and ESC-to-PGCLC differentiation. Specifically, Tex10-null spermatocytes arrest at metaphase I, compromising round spermatid formation. Tex10 depletion and overexpression compromise and enhance ESC-to-PGCLC differentiation, respectively. Mechanistically, bulk and single-cell RNA sequencing reveals that Tex10 depletion downregulates genes involved in pluripotency, PGC development, and spermatogenesis while upregulating genes promoting somatic programs. Chromatin occupancy study reveals that Tex10 binds to H3K4me3-marked promoters of Psmd3 and Psmd7, negative regulators of Wnt signaling, and activates their expression, thereby restraining Wnt signaling. Our study identifies Tex10 as a previously unappreciated factor in spermatogenesis and PGC development, offering potential therapeutic insights for treating male infertility.

ZFP541 and KCTD19 regulate chromatin organization and transcription programs for male meiotic progression

The successful progression of meiosis prophase I requires integrating information from the structural and molecular levels. In this study, we show that ZFP541 and KCTD19 work in the same genetic pathway to regulate the progression of male meiosis and thus fertility. The Zfp541 and/or Kctd19 knockout male mice show various structural and recombination defects including detached chromosome ends, aberrant localization of chromosome axis components and recombination proteins, and globally altered histone modifications. Further analyses on RNA-seq, ChIP-seq, and ATAC-seq data provide molecular evidence for the above defects and reveal that ZFP541/KCTD19 activates the expression of many genes by repressing several major transcription repressors. More importantly, we reveal an unexpected role of ZFP541/KCTD19 in directly modulating chromatin organization. These results suggest that ZFP541/KCTD19 simultaneously regulates the transcription cascade and chromatin organization to ensure the coordinated progression of multiple events at chromosome structural and biochemical levels during meiosis prophase I.

The pluripotency factor Tex10 finetunes Wnt signaling for PGC and male germline development

Testis-specific transcript 10 (Tex10) is a critical factor for pluripotent stem cell maintenance and preimplantation development. Here, we dissect its late developmental roles in primordial germ cell (PGC) specification and spermatogenesis using cellular and animal models. We discover that Tex10 binds the Wnt negative regulator genes, marked by H3K4me3, at the PGC-like cell (PGCLC) stage in restraining Wnt signaling. Depletion and overexpression of Tex10 hyperactivate and attenuate the Wnt signaling, resulting in compromised and enhanced PGCLC specification efficiency, respectively. Using the Tex10 conditional knockout mouse models combined with single-cell RNA sequencing, we further uncover critical roles of Tex10 in spermatogenesis with Tex10 loss causing reduced sperm number and motility associated with compromised round spermatid formation. Notably, defective spermatogenesis in Tex10 knockout mice correlates with aberrant Wnt signaling upregulation. Therefore, our study establishes Tex10 as a previously unappreciated player in PGC specification and male germline development by fine-tuning Wnt signaling.

Global phosphoproteomic analysis identified key kinases regulating male meiosis in mouse

Meiosis, a highly conserved process in organisms from fungi to mammals, is subjected to protein phosphorylation regulation. Due to the low abundance of phosphorylation, there is a lack of systemic characterization of phosphorylation regulation of meiosis in mammals. Using the phosphoproteomic approach, we profiled large-scale phosphoproteome of purified primary spermatocytes undergoing meiosis I, and identified 14,660 phosphorylation sites in 4419 phosphoproteins. Kinase-substrate phosphorylation network analysis followed by in vitro meiosis study showed that CDK9 was essential for meiosis progression to metaphase I and had enriched substrate phosphorylation sites in proteins involved in meiotic cell cycle. In addition, histones and epigenetic factors were found to be widely phosphorylated. Among those, HASPIN was found to be essential for male fertility. Haspin knockout led to misalignment of chromosomes, apoptosis of metaphase spermatocytes and a decreased number of sperm by deregulation of H3T3ph, chromosomal passenger complex (CPC) and spindle assembly checkpoint (SAC). The complicated protein phosphorylation and its important regulatory functions in meiosis indicated that in-depth studies of phosphorylation-mediated signaling could help us elucidate the mechanisms of meiosis.

SIX1 transcription factor: A review of cellular functions and regulatory dynamics

Sine Oculis Homeobox 1 (SIX1) is a member of homeobox transcription factor family having pivotal roles in organismal development and differentiation. This protein functionally acts to regulate the expression of different proteins that are involved in organ development during embryogenesis and in disorders like cancer. Aberrant expression of this homeoprotein has therefore been reported in multiple pathological complexities like hearing impairment and renal anomalies during development and tumorigenesis in adult life. Most of the cellular effects mediated by it are mostly due to its role as a transcription factor. This review presents a concise narrative of its structure, interaction partners and cellular functions vis a vis its role in cancer. We thoroughly discuss the reported molecular mechanisms that govern its function in cellular milieu. Its post-translational regulation by phosphorylation and ubiquitination are also discussed with an emphasis on yet to be explored mechanistic insights regulating its molecular dynamics to fully comprehend its role in development and disease.

Functions of cyclins and CDKs in mammalian gametogenesis†

Cyclins and cyclin-dependent kinases (CDKs) are key regulators of the cell cycle. Most of our understanding of their functions has been obtained from studies in single-cell organisms and mitotically proliferating cultured cells. In mammals, there are more than 20 cyclins and 20 CDKs. Although genetic ablation studies in mice have shown that most of these factors are dispensable for viability and fertility, uncovering their functional redundancy, CCNA2, CCNB1, and CDK1 are essential for embryonic development. Cyclin/CDK complexes are known to regulate both mitotic and meiotic cell cycles. While some mechanisms are common to both types of cell divisions, meiosis has unique characteristics and requirements. During meiosis, DNA replication is followed by two successive rounds of cell division. In addition, mammalian germ cells experience a prolonged prophase I in males or a long period of arrest in prophase I in females. Therefore, cyclins and CDKs may have functions in meiosis distinct from their mitotic functions and indeed, meiosis-specific cyclins, CCNA1 and CCNB3, have been identified. Here, we describe recent advances in the field of cyclins and CDKs with a focus on meiosis and early embryogenesis.

Identification of novel epigenetic abnormalities as sputum biomarkers for lung cancer risk among smokers and COPD patients

OBJECTIVES

Smoking is a common risk factor for chronic obstructive pulmonary disease (COPD) and lung cancer. Although COPD patients have higher risk of lung cancer compared to non-COPD smokers, the molecular links between these diseases are not well-defined. This study aims to identify genes that are downregulated by cigarette smoke and commonly repressed in COPD and lung cancer.

MATERIALS AND METHODS

Primary human airway epithelial cells (HAEC) were exposed to cigarette-smoke-extract (CSE) for 10-weeks and significantly suppressed genes were identified by transcriptome array. Epigenetic abnormalities of these genes in lung adenocarcinoma (LUAD) from patients with or without COPD were determined using genome-wide and gene-specific assays and by in vitro treatment of cell lines with trichostatin-A or 5-aza-2-deoxycytidine.

RESULTS

The ten most commonly downregulated genes following chronic CSE exposure of HAEC and show promoter hypermethylation in LUAD were selected. Among these, expression of CCNA1, SNCA, and ZNF549 was significantly reduced in lung tissues from COPD compared with non-COPD cases while expression of CCNA1 and SNCA was further downregulated in tumors with COPD. The promoter regions of all three genes were hypermethylated in LUAD but not normal or COPD lungs. The reduced expression and aberrant promoter hypermethylation of these genes in LUAD were independently validated using data from the Cancer Genome Atlas project. Importantly, SNCA and ZNF549 methylation detected in sputum DNA from LUAD (52% and 38%) cases were more prevalent compared to cancer-free smokers (26% and 15%), respectively (p < 0.02).

CONCLUSIONS

Our data show that suppression of CCNA1, SNCA, and ZNF549 in lung cancer and COPD occurs with or without promoter hypermethylation, respectively. Detecting methylation of these and previously identified genes in sputum of cancer-free smokers may serve as non-invasive biomarkers for early detection of lung cancer among high risk smokers including COPD patients.

Cyclin A1 in Oocytes Prevents Chromosome Segregation And Anaphase Entry

In several species, including Xenopus, mouse and human, two members of cyclin A family were identified. Cyclin A2, which is ubiquitously expressed in dividing cells and plays role in DNA replication, entry into mitosis and spindle assembly, and cyclin A1, whose function is less clear and which is expressed in spermatocytes, leukemia cells and in postmitotic multiciliated cells. Deletion of the gene showed that cyclin A1 is essential for male meiosis, but nonessential for female meiosis. Our results revealed, that the cyclin A1 is not only dispensable in oocytes, we show here that its expression is in fact undesirable in these cells. Our data demonstrate that the APC/C and proteasome in oocytes are unable to target sufficiently cyclin A1 before anaphase, which leads into anaphase arrest and direct inhibition of separase. The cyclin A1-induced cell cycle arrest is oocyte-specific and the presence of cyclin A1 in early embryos has no effect on cell cycle progression or chromosome division. Cyclin A1 is therefore not only an important cell cycle regulator with biased expression in germline, being essential for male and damaging for female meiosis, its persistent expression during anaphase in oocytes shows fundamental differences between APC/C function in oocytes and in early embryos.

SKP1 drives the prophase I to metaphase I transition during male meiosis

The meiotic prophase I to metaphase I (PI/MI) transition requires chromosome desynapsis and metaphase competence acquisition. However, control of these major meiotic events is poorly understood. Here, we identify an essential role for SKP1, a core subunit of the SKP1-Cullin-F-box (SCF) ubiquitin E3 ligase, in the PI/MI transition. SKP1 localizes to synapsed chromosome axes and evicts HORMAD proteins from these regions in meiotic spermatocytes. SKP1-deficient spermatocytes display premature desynapsis, precocious pachytene exit, loss of PLK1 and BUB1 at centromeres, but persistence of HORMAD, γH2AX, RPA2, and MLH1 in diplonema. Strikingly, SKP1-deficient spermatocytes show sharply reduced MPF activity and fail to enter MI despite treatment with okadaic acid. SKP1-deficient oocytes exhibit desynapsis, chromosome misalignment, and progressive postnatal loss. Therefore, SKP1 maintains synapsis in meiosis of both sexes. Furthermore, our results support a model where SKP1 functions as the long-sought intrinsic metaphase competence factor to orchestrate MI entry during male meiosis.

Di(2-ethylhexyl)phthalate induces reproductive toxicity via JAZF1/TR4 pathway and oxidative stress in pubertal male rats

Di(2-ethylhexyl)phthalate (DEHP) is a typical endocrine-disrupting chemical and reproductive toxicant. Although previous studies have attempted to describe the mechanism by which DEHP exposure results in reproductive dysfunction, few studies focused on puberty, a critical period of reproductive development, and the increased susceptibility to injury in adolescents. To elucidate the mechanism underpinning the testicular effects of DEHP in puberty, we sought to investigate the JAZF1/TR4 pathway in the testes of pubertal rats. Specifically, we focused on the role of the JAZF1/TR4 pathway in male reproduction, including the genes JAZF1, TR4, Sperm 1, and Cyclin A1. In the present study, rats were exposed to increasing concentrations of DEHP (0, 250, 500, and 1000 mg/kg/day) by oral gavages for 30 days. Then we assayed testicular zinc and oxidative stress levels. Our results indicated that DEHP exposure could lead to oxidative stress and decrease the contents of testicular zinc. Additionally, significant morphological changes and cell apoptosis were observed in testes exposed to DEHP, as identified by hematoxylin and eosin staining and the terminal deoxynucleotidyl transferase-mediated nick and labeling assay. By measuring the expression levels of the above relevant genes by qPCR, we found the DEHP-induced increased expression of JAZF1 and decreased expression of TR4, Sperm 1, and Cyclin A1. Therefore, we have demonstrated that in vivo exposure to DEHP might induce reproductive toxicity in pubertal male rats through the JAZF1/TR4 pathway and oxidative stress.

Emi2 Is Essential for Mouse Spermatogenesis

The meiotic functions of Emi2, an inhibitor of the APC/C complex, have been best characterized in oocytes where it mediates metaphase II arrest as a component of the cytostatic factor. We generated knockout mice to determine the in vivo functions of Emi2-in particular, its functions in the testis, where Emi2 is expressed at high levels. Male and female Emi2 knockout mice are viable but sterile, indicating that Emi2 is essential for meiosis but dispensable for embryonic development and mitotic cell divisions. We found that, besides regulating cell-cycle arrest in mouse eggs, Emi2 is essential for meiosis I progression in spermatocytes. In the absence of Emi2, spermatocytes arrest in early diplotene of prophase I. This arrest is associated with decreased Cdk1 activity and was partially rescued by a knockin mouse model of elevated Cdk1 activity. Additionally, we detected expression of Emi2 in spermatids and sperm, suggesting potential post-meiotic functions for Emi2.

Cyclin B1 is essential for mitosis in mouse embryos, and its nuclear export sets the time for mitosis

There is remarkable redundancy between the Cyclin-Cdk complexes that comprise the cell cycle machinery. None of the mammalian A-, D-, or E-type cyclins are required in development until implantation, and only Cdk1 is essential for early cell divisions. Cyclin B1 is essential for development, but whether it is required for cell division is contentious. Here, we used a novel imaging approach to analyze Cyclin B1-null embryos from fertilization onward. We show that Cyclin B1-/- embryos arrest in G2 phase after just two divisions. This is the earliest arrest of any Cyclin known and places Cyclin B1 with cdk1 as the essential regulators of the cell cycle. We reintroduced mutant proteins into this genetically null background to determine why Cyclin B1 is constantly exported from the nucleus. We found that Cyclin B1 must be exported from the nucleus for the cell to prevent premature entry to mitosis, and retaining Cyclin B1-Cdk1 at the plasma membrane precludes entry to mitosis.

Meiotic failure in cyclin A1-deficient mouse spermatocytes triggers apoptosis through intrinsic and extrinsic signaling pathways and 14-3-3 proteins

Cyclin A1 (Ccna1), a member of the mammalian A type cyclins, is most abundantly expressed in spermatocytes and is essential for spermatogenesis in the mouse. Ccna1- deficient spermatocytes arrest at late meiotic prophase and undergo apoptosis. To further delineate the mechanisms and key factors involved in this process, we have examined changes in expression of genes involved in both intrinsic and extrinsic signaling pathways that trigger apoptosis in the mutant spermatocytes. Our results show that both pathways are involved, and that the factors involved in the intrinsic pathway were expressed earlier than those involved in the extrinsic pathway. We have also begun to identify in vivo Ccna1-interacting proteins, using an unbiased biochemical approach, and identified 14-3-3, a key regulator of apoptosis, as a Ccna1-interacting protein. Expression levels of 14-3-3 proteins remain unchanged between wild type and mutant testes but there were differences in the subcellular distribution. In wild type control, 14-3-3 is detected in both cytosolic and nuclear fractions whereas it is restricted to the cytoplasm in mutant testes. This differential distribution of 14-3-3 may contribute to the induction of apoptosis in Ccna1-deficient spermatocytes. These results provide insight into the apoptotic mechanisms and pathways that are triggered when progression through the meiotic cell cycle is defective in male gametogenesis.

Arsenic-Induced Activation of the Homeodomain-Interacting Protein Kinase 2 (HIPK2) to cAMP-Response Element Binding Protein (CREB) Axis

Cyclic AMP-response element-binding protein (CREB) plays key transcriptional roles in cell metabolism, proliferation, and survival. Ser133 phosphorylation by protein kinase A (PKA) is a well-characterized CREB activation mechanism. Homeodomain-interacting protein kinase (HIPK) 2, a nuclear serine/threonine kinase, activates CREB through Ser271 phosphorylation; however, the regulatory mechanism remains uncharacterized. Transfection of CREB in HEK293 cells together with the kinase demonstrated that HIPK2 phosphorylated CREB at Ser271 but not Ser133; likewise, PKA phosphorylated CREB at Ser133 but not Ser271, suggesting two distinct CREB regulatory mechanisms by HIPK2 and PKA. In vitro kinase assay revealed that HIPK2, and HIPK1 and HIPK3, directly phosphorylated CREB. Cells exposed to 10μM sodium arsenite increased the stability of HIPK1 and HIPK2 proteins, leading to CREB activation via Ser271 phosphorylation. Phospho-Ser271 CREB showed facilitated interaction with the TFIID subunit coactivator TAF4 assessed by immunoprecipitation. Furthermore, a focused gene array between cells transfected with CREB alone and CREB plus HIPK2 over empty vector-transfected control displayed 14- and 32-fold upregulation of cyclin A1, respectively, while no upregulation was displayed by HIPK2 alone. These results suggest that the HIPK2-phospho-Ser271 CREB axis is a new arsenic-responsive CREB activation mechanism in parallel with the PKA-phospho-Ser133 CREB axis.

Disrupting Cyclin Dependent Kinase 1 in Spermatocytes Causes Late Meiotic Arrest and Infertility in Mice

While cyclin dependent kinase 1 (CDK1) has a critical role in controlling resumption of meiosis in oocytes, its role has not been investigated directly in spermatocytes. Unique aspects of male meiosis led us to hypothesize that its role is different in male meiosis than in female meiosis. We generated a conditional knockout (cKO) of the Cdk1 gene in mouse spermatocytes to test this hypothesis. We found that CDK1-null spermatocytes undergo synapsis, chiasmata formation, and desynapsis as is seen in oocytes. Additionally, CDK1-null spermatocytes relocalize SYCP3 to centromeric foci, express H3pSer10, and initiate chromosome condensation. However, CDK1-null spermatocytes fail to form condensed bivalent chromosomes in prophase of meiosis I and instead are arrested at prometaphase. Thus, CDK1 has an essential role in male meiosis that is consistent with what is known about the role of CDK1 in female meiosis, where it is required for formation of condensed bivalent metaphase chromosomes and progression to the first meiotic division. We found that cKO spermatocytes formed fully condensed bivalent chromosomes in the presence of okadaic acid, suggesting that cKO chromosomes are competent to condense, although they do not do so in vivo. Additionally, arrested cKO spermatocytes exhibited irregular cell shape, irregular large nuclei, and large distinctive nucleoli. These cells persist in the seminiferous epithelium through the next seminiferous epithelial cycle with a lack of stage XII checkpoint-associated cell death. This indicates that CDK1 is required upstream of a checkpoint-associated cell death as well as meiotic metaphase progression in mouse spermatocytes.

Anti-Colon Cancer Effects of 6-Shogaol Through G2/M Cell Cycle Arrest by p53/p21-cdc2/cdc25A Crosstalk

Chemopreventive agents can be identified from botanicals. Recently, there has been strong support for the potential of 6-shogaol, a natural compound from dietary ginger (Zingiber officinale), in cancer chemoprevention. However, whether 6-shogaol inhibits the growth of colorectal tumors in vivo remains unknown, and the underlying anticancer mechanisms have not been well characterized. In this work, we observed that 6-shogaol (15 mg/kg) significantly inhibited colorectal tumor growth in a xenograft mouse model. We show that 6-shogaol inhibited HCT-116 and SW-480 cell proliferation with IC50 of 7.5 and 10 μM, respectively. Growth of HCT-116 cells was arrested at the G2/M phase of the cell cycle, primarily mediated by the up-regulation of p53, the CDK inhibitor p21(waf1/cip1) and GADD45α, and by the down-regulation of cdc2 and cdc25A. Using p53(-/-) and p53(+/+) HCT-116 cells, we confirmed that p53/p21 was the main pathway that contributed to the G2/M cell cycle arrest by 6-shogaol. 6-Shogaol induced apoptosis, mainly through the mitochondrial pathway, and the bcl-2 family might act as a key regulator. Our results demonstrated that 6-shogaol induces cancer cell death by inducing G2/M cell cycle arrest and apoptosis. 6-Shogaol could be an active natural product in colon cancer chemoprevention.

Transcriptome profiling of the testis reveals genes involved in spermatogenesis and marker discovery in the oriental fruit fly, Bactrocera dorsalis

The testis is a highly specialized tissue that plays a vital role in ensuring fertility by producing spermatozoa, which are transferred to the female during mating. Spermatogenesis is a complex process, resulting in the production of mature sperm, and involves significant structural and biochemical changes in the seminiferous epithelium of the adult testis. The identification of genes involved in spermatogenesis of Bactrocera dorsalis (Hendel) is critical for a better understanding of its reproductive development. In this study, we constructed a cDNA library of testes from male B. dorsalis adults at different ages, and performed de novo transcriptome sequencing to produce a comprehensive transcript data set, using Illumina sequencing technology. The analysis yielded 52 016 732 clean reads, including a total of 4.65 Gb of nucleotides. These reads were assembled into 47 677 contigs (average 443 bp) and then clustered into 30 516 unigenes (average 756 bp). Based on BLAST hits with known proteins in different databases, 20 921 unigenes were annotated with a cut-off E-value of 10(-5). The transcriptome sequences were further annotated using the Clusters of Orthologous Groups, Gene Orthology and the Kyoto Encyclopedia of Genes and Genomes databases. Functional genes involved in spermatogenesis were analysed, including cell cycle proteins, metalloproteins, actin, and ubiquitin and antihyperthermia proteins. Several testis-specific genes were also identified. The transcripts database will help us to understand the molecular mechanisms underlying spermatogenesis in B. dorsalis. Furthermore, 2913 simple sequence repeats and 151 431 single nucleotide polymorphisms were identified, which will be useful for investigating the genetic diversity of B. dorsalis in the future.

Cyclin A1 expression predicts progression in pT1 urothelial carcinoma of bladder: a tissue microarray study of 149 patients treated by transurethral resection

AIMS

To evaluate the immunoexpression of cyclin A1 in pT1 urothelial carcinomas of the bladder (UC) from a cohort of patients treated by transurethral resection of the bladder (TURB), to determine its value in predicting tumour recurrence, tumour progression, or systemic metastases.

METHODS AND RESULTS

Five tissue microarrays (TMAS) were constructed from representative paraffin blocks of high-grade pT1 UC from 149 consecutive patients. Cyclin A1 immunoexpression was evaluated as the percentage of tumour cells with positive nuclear staining estimated at each TMA spot. The cutoff for cyclin A1 positivity was set at 10% of cells. Outcome variables included tumour recurrence and tumour progression as the primary endpoints. Cyclin A1 positivity was associated with tumour progression but not with tumour recurrence or the presence of adjacent carcinoma in situ in the biopsy. Also, patients with pT1b at biopsy and cyclin A1 expression showed higher progression rates than patients with pT1a at biopsy and without cyclin A1 expression, respectively. Combining pT1 stage at biopsy and cyclin A1 expression more accurately predicted tumour progression than pT1 stage at biopsy alone and cyclin A1 expression alone.

CONCLUSIONS

Cyclin A1 immunoexpression is of potential utility in predicting disease progression in patients with pT1 UC.

The APC/C activator FZR1 is essential for meiotic prophase I in mice

Fizzy-related 1 (FZR1) is an activator of the Anaphase promoting complex/cyclosome (APC/C) and an important regulator of the mitotic cell division cycle. Using a germ-cell-specific conditional knockout model we examined its role in entry into meiosis and early meiotic events in both sexes. Loss of APC/C(FZR1) activity in the male germline led to both a mitotic and a meiotic testicular defect resulting in infertility due to the absence of mature spermatozoa. Spermatogonia in the prepubertal testes of such mice had abnormal proliferation and delayed entry into meiosis. Although early recombination events were initiated, male germ cells failed to progress beyond zygotene and underwent apoptosis. Loss of APC/C(FZR1) activity was associated with raised cyclin B1 levels, suggesting that CDK1 may trigger apoptosis. By contrast, female FZR1Δ mice were subfertile, with premature onset of ovarian failure by 5 months of age. Germ cell loss occurred embryonically in the ovary, around the time of the zygotene-pachytene transition, similar to that observed in males. In addition, the transition of primordial follicles into the growing follicle pool in the neonatal ovary was abnormal, such that the primordial follicles were prematurely depleted. We conclude that APC/C(FZR1) is an essential regulator of spermatogonial proliferation and early meiotic prophase I in both male and female germ cells and is therefore important in establishing the reproductive health of adult male and female mammals.

RanBPM, a scaffolding protein for gametogenesis

RanBPM is a multimodular scaffold protein that interacts with a great variety of molecules including nuclear, cytoplasmic, and membrane proteins. By building multiprotein complexes, RanBPM is thought to regulate various signaling pathways, especially in the immune and nervous system. However, the diversity of these interactions does not facilitate the identification of its precise mechanism of action, and therefore the physiological role of RanBPM still remains unclear. Recently, RanBPM has been shown to be critical for the fertility of both genders in mouse. Although mechanistically it is still unclear how RanBPM affects gametogenesis, the data collected so far suggest that it is a key player in this process. Here, we examine the RanBPM sterility phenotype in the context of other genetic mutations affecting mouse gametogenesis to investigate whether this scaffold protein affects the function of other known proteins whose deficiency results in similar sterility phenotypes.

Role of cyclins in controlling progression of mammalian spermatogenesis

Cyclins are key regulators of the mammalian cell cycle, functioning primarily in concert with their catalytic partners, the cyclin-dependent kinases (Cdks). While their function during mitosis in somatic cells has been extensively documented, their function during both mitosis and meiosis in the germ line is poorly understood. From the perspective of cell cycle regulation there are several aspects of mammalian spermatogenesis that suggest unique modes of regulation and hence, possible unique functions for the cyclins. This review will summarize our current understanding of cyclin expression and function in the male germ line, with particular focus on the A and E type cyclins in the mouse model. While the focus is on mammalian spermatogenesis, we note contrasts with similar functions in the female germ line when relevant and also draw upon observations in other model systems to provide further insight.

Decreased expression of LATS1 is correlated with the progression and prognosis of glioma

Background

LATS1 is a tumor suppressor genes implicated in the pathogenesis of certain types of tumors, but its role is not known in human glioma.

Methods

Using real-time PCR and immunohistochemistry, we detected the mRNA and protein expression of LATS1 in glioma. The effect of LATS1 on cell growth and invasion were investigated.

Results

We found that mRNA and protein of LATS1 expression is significantly downregulated in glioma compared with normal control brain tissues. Furthermore, reduced LATS1 expression was markedly negatively correlated with WHO grade and KPS (p<0.001 and p<0.001) in glioma patients. Patients with lower LATS1 expression had a significantly shorter overall survival time than did patients with higher LATS1 expression. Multivariate analysis suggested that the level of LATS1 expression was an independent prognostic indicator (p<0.001) for the survival of patients with glioma. Forced expression of LATS1 in glioma U251 cells not only significantly suppressed cell growth, migration and invasion, but retarded cell cycle progression from G2/M to G1 in vitro. Finally, we found that overexpressed LATS1 markedly inhibited the expression level of cell cycle factor CCNA1.

Conclusion

These results indicate that LATS1 is an important candidate tumor suppressor and its downregulated expression may contribute to glioma progression.

The ubiquitin-specific protease USP2a enhances tumor progression by targeting cyclin A1 in bladder cancer

The deubiquitinating enzyme USP2a has shown oncogenic properties in many cancer types by impairing ubiquitination of FASN, MDM2, MDMX or Aurora A. Aberrant expression of USP2a has been linked to progression of human tumors, particularly prostate cancer. However, little is known about the role of USP2a or its mechanism of action in bladder cancer. Here, we provide evidence that USP2a is an oncoprotein in bladder cancer cells. Enforced expression of USP2a caused enhanced proliferation, invasion, migration and resistance to several chemotherapeutic reagents, while USP2a loss resulted in slower proliferation, greater chemosensitivity and reduced migratory/invasive capability compared with control cells. USP2a, but not a catalytically inactive mutant, enhanced proliferation in immortalized TRT-HU1 normal human bladder epithelial cells. USP2a bound to cyclin A1 and prevented cyclin A1 ubiquitination, leading to accumulation of cyclin A1 by a block in degradation. Enforced expression of wild type USP2a, but not an inactive USP2a mutant, resulted in cyclin A1 accumulation and increased cell proliferation. We conclude that USP2a impairs ubiquitination and stabilizes an important cell cycle regulator, cyclin A1, raising the possibility of USP2a targeting as a therapeutic strategy against bladder tumors in combination with chemotherapy.

Function of the A-type cyclins during gametogenesis and early embryogenesis

The cyclins and their cyclin-dependent kinase partners, the Cdks, are the basic components of the machinery that regulates the passage of cells through the cell cycle. Among the cyclins, those known as the A-type cyclins are unique in that in somatic cells, they appear to function at two stages of the cell cycle, at the G1-S transition and again as the cells prepare to enter M-phase. Higher vertebrate organisms have two A-type cyclins, cyclin A1 and cyclin A2, both of which are expressed in the germ line and/or early embryo, following highly specialized patterns that suggest functions in both mitosis and meiosis. Insight into their in vivo functions has been obtained from gene targeting experiments in the mouse model. Loss of cyclin A1 results in disruption of spermatogenesis and male sterility due to cell arrest in the late diplotene stage of the meiotic cell cycle. In contrast, cyclin A2-deficiency is marked by early embryonic lethality; thus, understanding the function of cyclin A2 in the adult germ line awaits conditional mutagenesis or other approaches to knock down its expression.

Immunohistochemical expression of cyclin A in testicular biopsies of fertile and infertile men: correlation with the morphometry of seminiferous tubules

Cyclin A is a member of the cyclin family of proteins, which are required for both the mitotic and meiotic divisions that characterise spermatogenesis in human and other mammalian species. The data on cyclin A expression in various human spermatogenic disorders and its relationship to the morphology of seminiferous tubules are not well clarified. This study aimed to evaluate the immunohistochemical expression of cyclin A in testicular biopsies of different spermatogenic disorders correlating with the morphology of seminiferous tubules using morphometry tools. Immunohistochemical evaluation of cyclin A was carried out on testicular biopsies obtained from 48 infertile males (nonobstructive azoospermia) and 15 normal subjects together with using semiautomatic morphometric analysis for evaluation of seminiferous tubules. Cyclin A is expressed in 100% of normal and hypospermatogenesis groups and in 80% of maturation arrest group, with complete absence in Sertoli cell only group. In positive cases, cyclin A stained the nuclei of spermatogonia and primary spermatocytes with a higher intensity of expression in normal cases compared with infertile group. Cyclin A expression was significantly associated with the different examined morphometric parameters. Cyclin A is involved in both mitosis and meiosis of human spermatogenesis as it is expressed in spermatogonia and primary spermatocytes. Morphometry of human testis is intimately correlated with the testicular histopathology.

Regulating mitosis and meiosis in the male germ line: critical functions for cyclins

Key components of the cell cycle machinery are the regulatory subunits, the cyclins, and their catalytic partners the cyclin-dependent kinases. Regulating the cell cycle in the male germ line cells represents unique challenges for this machinery given the constant renewal of gametes throughout the reproductive lifespan and the induction of the unique process of meiosis, a highly specialized kind of cell division. With challenges come opportunities to the critical eye, recognizing that understanding these specialized modes of regulation will provide considerable insight into both normal differentiation as well as disease conditions, including infertility and oncogenesis.

MPF governs the assembly and contraction of actomyosin rings by activating RhoA and MAPK during chemical-induced cytokinesis of goat oocytes

The interplay between maturation-promoting factor (MPF), mitogen-activated protein kinase (MAPK) and Rho GTPase during actin-myosin interactions has yet to be determined. The mechanism by which microtubule disrupters induce the formation of ooplasmic protrusion during chemical-assisted enucleation of mammalian oocytes is unknown. Moreover, a suitable model is urgently needed for the study of cytokinesis. We have established a model of chemical-induced cytokinesis and have studied the signaling events leading to cytokinesis using this model. The results suggested that microtubule inhibitors activated MPF, which induced actomyosin assembly (formation of ooplasmic protrusion) by activating RhoA and thus MAPK. While MAPK controlled actin recruitment on its own, MPF promoted myosin enrichment by activating RhoA and MAPK. A further chemical treatment of oocytes with protrusions induced constriction of the actomyosin ring by inactivating MPF while activating RhoA. In conclusion, the present data suggested that the assembly and contraction of the actomyosin ring were two separable steps: while an increase in MPF activity promoted the assembly through RhoA-mediated activation of MAPK, a decrease in MPF activity triggered contraction of the ring by activating RhoA.

KDM8, a H3K36me2 histone demethylase that acts in the cyclin A1 coding region to regulate cancer cell proliferation

Localized chromatin modifications of histone tails play an important role in regulating gene transcription, and aberration of these processes leads to carcinogenesis. Methylated histone lysine residues, a key player in chromatin remodeling, are demethylated by the JmjC class of enzymes. Here we show that JMJD5 (now renamed KDM8), a JmjC family member, demethylates H3K36me2 and is required for cell cycle progression. Chromatin immunoprecipitation assays applied to human genome tiling arrays in conjunction with RNA microarray revealed that KDM8 occupies the coding region of cyclin A1 and directly regulates transcription. Mechanistic analyses showed that KDM8 functioned as a transcriptional activator by inhibiting HDAC recruitment via demethylation of H3K36me2, an epigenetic repressive mark. Tumor array experiments revealed KDM8 is overexpressed in several types of cancer. In addition, loss-of-function studies in MCF7 cells leads to cell cycle arrest. These studies identified KDM8 as an important cell cycle regulator.

Mutation of Eif4g3, encoding a eukaryotic translation initiation factor, causes male infertility and meiotic arrest of mouse spermatocytes

The ENU-induced repro8 mutation was identified in a screen to uncover genes that control mouse gametogenesis. repro8 causes male-limited infertility, with failure of spermatocytes to exit meiotic prophase via the G2/MI transition. The repro8 mutation is in the Eif4g3 gene, encoding eukaryotic translation initiation factor 4, gamma 3. Mutant germ cells appear to execute events of meiotic prophase normally, and many proteins characteristic of the prophase-to-metaphase transition are not obviously depleted. However, activity of CDC2A (CDK1) kinase is dramatically reduced in mutant spermatocytes. Strikingly, HSPA2, a chaperone protein for CDC2A kinase, is absent in mutant spermatocytes in spite of the presence of Hspa2 transcript, consistent with the observation that the repro8 phenotype is markedly similar to the phenotype of the Hspa2 knockout. Thus, EIF4G3 is required for HSPA2 translation in spermatocytes, a finding that provides the first genetic evidence for selective translational control of meiotic exit in mammalian spermatocytes.

Mutations of the cyclin A1 gene are not a common cause of male infertility

Cyclin A1 is essential for meiosis as shown by its essential role in mouse spermatogenesis, suggesting that changes in the gene may also alter male fertility in humans. In the present study, we performed a mutation screening of the cyclin A1 gene in order to investigate the possible association between the mutations of the gene and human impaired spermatogenesis using denaturing high performance liquid chromatography (DHPLC) in 347 infertile patients with azoospermia or severe oligozoospermia and 210 fertile controls. Four point mutations, c.321T>C, IVS3 +32G>C, IVS5+38A>G and c.1158G>A, were identified, but no association of these with spermatogenesis impairment was detected, suggesting that these cyclin A1 gene mutations are unlikely a common genetic cause for impaired human spermatogenesis.

The seminiferous epithelium cycle and its duration in different breeds of dog (Canis familiaris)

Testis structure and function in dogs are relatively poorly investigated. The aim of the present study was to carry out a comparative investigation of the stages of the seminiferous epithelium cycle and its duration in different breeds of dog. Fifty-six sexually mature dogs (mongrel, n = 12; pinscher, n = 12; beagle, n = 5; American pit bull, n = 9; poodle, n = 12; and Labrador retriever, n = 6) were analysed. Intratesticular injections of tritiated thymidine were given to determine the duration of spermatogenesis. Orchiectomy was performed at different time periods following injection (1 h, 2 and 4 weeks). Testis fragments were embedded in plastic and routinely prepared for histological and autoradiographic evaluations. Eight stages were characterized based on the acrosome system. Significant (P < 0.05) differences were found for the frequencies of the different stages characterized (except Stages V, VI and VIII), particularly for the mongrel. Stage IV (when spermiation occurs) was the most frequent in all six breeds (~25%), whereas Stages II and VIII were the least frequent (< 8%). Each spermatogenic cycle and the total duration of spermatogenesis lasted 13.73 +/- 0.03 and 61.9 +/- 0.14 days, respectively, for the mongrel, poodle, pinscher, beagle, and Labrador retriever. These values were approximately 10% lower (P < 0.03) for the American pit bull (12.55 +/- 0.26 and 56.5 +/- 1.17 days, respectively). To our knowledge, this is the first comprehensive study to perform a careful investigation of stage frequencies and seminiferous epithelium cycle duration in this very important domestic species.

Linker histones stimulate HSPA2 ATPase activity through NASP binding and inhibit CDC2/Cyclin B1 complex formation during meiosis in the mouse

In mammalian spermatocytes, cell division cycle protein 2 (CDC2)/cyclin B1 and the chaperone heat shock protein A2 (HSPA2) are required for the G2-->M transition in prophase I. Here, we demonstrate that in primary spermatocytes, linker histone chaperone testis/embryo form of nuclear autoantigenic sperm protein (tNASP) binds the heat shock protein HSPA2, which localizes on the synaptonemal complex of spermatocytes. Significantly, the tNASP-HSPA2 complex binds linker histones and CDC2, forming a larger complex. We demonstrate that increasing amounts of tNASP favor tNASP-HSPA2-CDC2 complex formation. Binding of linker histones to tNASP significantly increases HSPA2 ATPase activity and the capacity of tNASP to bind HSPA2 and CDC2, precluding CDC2/cyclin B1 complex formation and, consequently, decreasing CDC2/cyclin B1 kinase activity. Linker histone binding to NASP controls the ability of HSPA2 to activate CDC2 for CDC2/cyclin B1 complex formation; therefore, tNASP's role is to provide the functional link between linker histones and cell cycle progression during meiosis.

Expression of Six1 homeobox gene during development of the mouse submandibular salivary gland

BACKGROUND

Members of the Six family of homeoproteins are expressed in numerous tissues during vertebrate embryogenesis, and are critical regulators of both cell proliferation and survival. Here we report the temporal and spatial expression of Six1 during maturation of the mouse submandibular salivary gland (SSG) from embryonic day 18.5 (E18.5) to postnatal day 28. Additionally, we examine the role of Six1 during SSG development using Six1-deficient mice.

METHODS

Six1 expression was assessed by reverse transcription-polymerase chain reaction, Western blot, and immunofluorescence. Proliferation was measured by bromodeoxyuridine (BrdU) incorporation index, and apoptosis was evaluated by TUNEL assay.

RESULTS

Six1 mRNA and protein levels are high in the epithelial SSG cells at E18.5 and decrease progressively in the postnatal maturing SSG. Although SSGs from Six1(-/-) embryos are significantly smaller than wild type SSGs, the histological structures of the SSG acini and ducts are similar. Six1(-/-) salivary epithelial cells exhibit an intrinsic defect in cell proliferation accompanied by a significant reduction in the Six1 target gene cyclin A1, previously shown to be a critical mediator of Six1-induced proliferation.

CONCLUSION

Our results suggest that the reduction in size of Six1(-/-) SSGs is result of a decrease in cell proliferation during development/maturation.

The sine oculis homeobox (SIX) family of transcription factors as regulators of development and disease

The sine oculis homeobox (SIX) protein family is a group of evolutionarily conserved transcription factors that are found in diverse organisms that range from flatworms to humans. These factors are expressed within, and play pivotal developmental roles in, cell populations that give rise to the head, retina, ear, nose, brain, kidney, muscle and gonads. Mutations within the fly and mammalian versions of these genes have adverse consequences on the development of these organs/tissues. Several SIX proteins have been shown to directly influence the cell cycle and are present at elevated levels during tumorigenesis and within several cancers. This review aims to highlight aspects of (1) the evolutionary history of the SIX family; (2) the structural differences and similarities amongst the different SIX proteins; (3) the role that these genes play in retinal development; and (4) the influence that these proteins have on cell proliferation and growth.

Distinct properties of cyclin-dependent kinase complexes containing cyclin A1 and cyclin A2

The distinct expression patterns of the two A-type cyclins during spermatogenesis and the absolute requirement for cyclin A1 in this biological process in vivo suggest that they may confer distinct biochemical properties to their CDK partners. We therefore compared human cyclin A1- and cyclin A2-containing CDK complexes in vitro by determining kinetic constants and by examining the complexes for their ability to phosphorylate pRb and p53. Differences in biochemical activity were observed in CDK2 but not CDK1 when complexed with cyclin A1 versus cyclin A2. Further, CDK1/cyclin A1 is a better kinase complex for phosphorylating potentially physiologically relevant substrates pRb and p53 than CDK2/cyclin A2. The activity of CDKs can therefore be regulated depending upon which A-type cyclin they bind and CDK1/cyclin A1 might be preferred in vivo.

A novel testis protein, RSB-66, interacting with INCA1 (inhibitor of Cdk interacting with cyclin A1)

rsb-66 is a novel gene from a suppression subtracted hybridization (SSH) library of round spermatid-specific cDNAs against those of primary spermatocytes. It was found to be specifically expressed in round spermatids. To explore the function of RSB-66, a yeast two-hybrid system was used to screen for potential interacting partners in a human testis cDNA library. HSD45, also known as INCA1 (inhibitor of Cdk interacting with cyclin A1), was identified as one of the positive clones. The interaction between RSB-66 and INCA1 was demonstrated to occur by GST pull down and coimmunoprecipitation. Using immunofluorescence, RSB-66 was found to be specifically expressed in round spermatids, mainly in the cytoplasm. When being transfected into HeLa cells, RSB-66 and INCA1 were found to be co-localized principally in the cytoplasm. The alpha helix in the RSB-66 C terminal and two amino acid residues (tyr117 and his119) appear to be crucial for its function.

Function of cyclins in regulating the mitotic and meiotic cell cycles in male germ cells

The specialized cell cycles that characterize various aspects of the differentiation of germ cells provide a unique opportunity to understand heretofore elusive aspects of the in vivo function of cell cycle regulators. Key components of the cell cycle machinery are the regulatory sub-units, the cyclins, and their catalytic partners, the cyclin-dependent kinases. Some of the cyclins exhibit unique patterns of expression in germ cells that suggest possible concomitant distinct functions, predictions that are being explored by targeted mutagenesis in mouse models. A novel, meiosis-specific function has been shown for one of the A-type cyclins, cyclin A1. Embryonic lethality has obviated understanding of the germline functions of cyclin A2 and cyclin B1, while yet other cyclins, although expressed at specific stages of germ cell development, may have less essential function in the male germline.

Regulation of the meiotic prophase I to metaphase I transition in mouse spermatocytes

The meiotic prophase I to metaphase I transition (G2/MI) involves disassembly of synaptonemal complex (SC), chromatin condensation, and final compaction of morphologically distinct MI bivalent chromosomes. Control of these processes is poorly understood. The G2/MI transition was experimentally induced in mouse pachytene spermatocytes by okadaic acid (OA), and kinetic analysis revealed that disassembly of the central element of the SC occurred very rapidly after OA treatment, before histone H3 phosphorylation on Ser10. These events were followed by relocalization of SYCP3 and final condensation of bivalents. Enzymatic control of these G2/MI transition events was studied using small molecule inhibitors: butyrolactone I (BLI), an inhibitor of cyclin-dependent kinases (CDKs) and ZM447439 (ZM), an inhibitor of aurora kinases (AURKs). The formation of highly condensed MI bivalents and disassembly of the SC are regulated by both CDKs and AURKs. AURKs also mediate phosphorylation of histone H3 in meiosis. However, neither BLI nor ZM inhibited disassembly of the central element of the SC. Thus, despite evidence that the metaphase promoting factor is a universal regulator of the onset of cell division, desynapsis, the first and key step of the G2/MI transition, occurs independently of BLI-sensitive CDKs and ZM-sensitive AURKs.

Involvement of cyclins in mammalian spermatogenesis

Mammalian spermatogenesis is a complicated developmental process by which undifferentiated germ cells continuously produce mature sperm throughout a lifetime. Stringent control of the cell cycle during spermatogenesis is required to ensure self-renewal of male germ line cells and differentiation of appropriate numbers of cells for the various lineages. Cyclins are key factors of cell cycle regulation and play crucial roles in governing both the mitotic and meiotic divisions that characterize spermatogenesis. Abnormal expression of some types of cyclins in the testes can induce apoptosis, infertility, testicular tumors, and other problems related to spermatogenesis in mammals. In this review, available data regarding cellular and molecular regulation of several different types of cyclins during mammalian spermatogenesis are collected and further discussed.

Cyclin A1-deficient mice lack histone H3 serine 10 phosphorylation and exhibit altered aurora B dynamics in late prophase of male meiosis

Male mice lacking cyclin A1 protein are sterile. Their sterility results from an arrest in the meiotic cell cycle of spermatocytes, which we now identify as occurring at late diplotene, immediately before diakinesis. The stage of arrest in cyclin A1-deficient mice is distinct from the arrest seen in spermatocytes that are deficient in its putative catalytic partner Cdk2, which occurs much earlier in pachytene. The arrest in cyclin A1-deficient spermatocytes is also accompanied by an unusual clustering of centromeric heterochromatin. Consistent with a possible defect in the centromeric region, immunofluorescent staining of cyclin A1 protein shows localization in the region of the centromere. Phosphorylation of histone H3 at serine 10 in pericentromeric heterochromatin, which normally occurs in late diplotene, is reduced in spermatocytes from heterozygous Ccna1(+/-) testes and completely absent in spermatocytes with no cyclin A1 protein. Concomitantly, the levels of pericentromeric aurora B kinase, known to phosphorylate histone H3 during meiosis, are partially reduced in spermatocytes from testes of heterozygous mice and further reduced in homozygous null spermatocytes. These data suggest a critical and concentration-dependent function for cyclin A1 in the pericentromeric region in late diplotene of meiosis, perhaps in assembly or function of the passenger protein complex.

Distinct roles for the mammalian A-type cyclins during oogenesis

There are two A-type cyclins in higher vertebrates, cyclin A1 and A2. Targeted mutagenesis has shown that cyclin A2 is essential for early embryonic development while cyclin A1 is required only for male meiosis. The embryonic lethality of cyclin A2 knockout mice has obviated understanding its role in other aspects of mammalian development, including the germ line. We reported previously that cyclin A2 expression in the male germ line is consistent with a role in both mitotic and meiotic cell cycles. Using in situ hybridization and immunohistochemistry, we now observe high levels of cyclin A2 in granulosa cells and less-abundant but readily detectable expression in ovarian and ovulated oocytes. A decrease in cyclin A2 protein was observed in oocytes from embryonic stages to post-natal and adult ovaries. Interestingly, cyclin A2 protein was nuclear in oocytes from embryonic day 13.5 to 15.5, changing to largely cytoplasmic in oocytes from embryonic day 16.5 to post-natal and adults. Readily detectable expression of the cyclin-dependent kinases Cdk1 and Cdk2, two common partners for the A-type cyclins, was observed in granulosa cells and oocytes at all stages of folliculogenesis. Cdk1 was predominantly cytoplasmic, whereas Cdk2 was both cytoplasmic and nuclear in oocytes. No cyclin A1 expression, at either the mRNA level or the protein level was detected in either embryonic or adult ovaries, consistent with the full fertility observed in female cyclin A1-deficient mice. These results suggest that in the female germ line, cyclin A2 but not cyclin A1 has distinct roles in both mitosis and meiosis.

Cyclin A1 expression and associations with disease characteristics in childhood acute lymphoblastic leukemia

A critical cell cycle regulatory protein, cyclin A1, has been implicated in the development of acute myeloid leukemia (AML). Here, we have examined the expression and clinical significance of cyclin A1 in childhood acute lymphoblastic leukemia (ALL). Cyclin A1 was highly expressed in lymphoblastic leukemic cell lines and in 22 of 30 ALL patients (73%). Cyclin A1 expression correlated with patient age (P=0.006), but not with cytogenetic abnormalities. Patients with high levels of cyclin A1 had poorer event-free survival (57.9%) compared to patients with lower levels (75%).

Increased expression of cyclin A1 protein is associated with all-trans retinoic acid-induced apoptosis

Deregulated cell growth and inhibition of apoptosis are hallmarks of cancer. All-trans retinoic acid induces clinical remission in patients with acute promyelocytic leukemia by inhibiting cell growth and inducing differentiation and apoptosis of the leukemic blasts. An important role of the cell cycle regulatory protein, cyclin A1, in the development of acute myeloid leukemia has previously been demonstrated in a transgenic mouse model. We have recently shown that there was a direct interaction between cyclin A1 and a major all-trans retinoic acid receptor, RAR alpha, following all-trans retinoic acid treatment of leukemic cells. In the present study, we investigated whether cyclin A1 might be involved in all-trans retinoic acid-induced apoptosis in U-937 leukemic cells. We found that all-trans retinoic acid-induced apoptosis was associated with concomitant increase in cyclin A1 expression. However, there was no induction of cyclin A1 mRNA expression following the all-trans retinoic acid-induced apoptosis. Treatment of cells with a caspase inhibitor was not able to prevent all-trans retinoic acid-induced up-regulation of cyclin A1 expression. Interestingly, induced cyclin A1 expression in U-937 cells led to a significant increase in the proportion of apoptotic cells. Further, U-937 cells overexpressing cyclin A1 appeared to be more sensitive to all-trans retinoic acid-induced apoptosis indicating the ability of cyclin A1 to mediate all-trans retinoic acid-induced apoptosis. Induced cyclin E expression was not able to initiate cell death in U-937 cells. Our results indicate that cyclin A1 might have a role in apoptosis by mediating all-trans retinoic acid-induced apoptosis.

Cell cycle regulation in mammalian germ cells

Meiosis is a unique form of cellular division by which a diploid cell produces genetically distinct haploid gametes. Initiation and regulation of mammalian meiosis differs between the sexes. In females, meiosis is initiated during embryo development and arrested shortly after birth during prophase I. In males, spermatogonial stem cells initiate meiosis at puberty and proceed through gametogenesis with no cell cycle arrest. Mouse genes required for early meiotic cell cycle events are being identified by comparative analysis with other eukaryotic systems, by virtue of gene knockout technology and by mouse mutagenesis screens for reproductive defects. This review focuses on mouse reproductive biology and describes the available mouse mutants with defects in the early meiotic cell cycle and prophase I regulatory events. These research tools will permit rapid advances in such medically relevant research areas as infertility, embryo lethality and developmental abnormalities.

Not all germ cells are created equal: Aspects of sexual dimorphism in mammalian meiosis

The study of mammalian meiosis is complicated by the timing of meiotic events in females and by the intermingling of meiotic sub-stages with somatic cells in the gonad of both sexes. In addition, studies of mouse mutants for different meiotic regulators have revealed significant differences in the stringency of meiotic events in males versus females. This sexual dimorphism implies that the processes of recombination and homologous chromosome pairing, while being controlled by similar genetic pathways, are subject to different levels of checkpoint control in males and females. This review is focused on the emerging picture of sexual dimorphism exhibited by mammalian germ cells using evidence from the broad range of meiotic mutants now available in the mouse. Many of these mouse mutants display distinct differences in meiotic progression and/or dysfunction in males versus females, and their continued study will allow us to understand the molecular basis for the sex-specific differences observed during prophase I progression.