Distinct patterns of expression of the D-type cyclins during testicular development in the mouse

Pitx2 suppression at meiotic stages associates with seasonal inhibition of testis development in Rattus norvegicus caraco

The bicoid-related transcription factor 2 (Pitx2) plays a crucial role in the development of many organs and tissues by affecting the mitotic cell cycle. Postnatal testis development is related to mitosis and meiosis in multiple cell types, but the role of Pitx2 gene in seasonal inhibition of testicular development remains unknown in rodents. We analyzed PITX2 protein and Pitx2 mRNA expression features using both laboratory and wild male Rattus norvegicus caraco. In postnatal testicle of laboratory colony, we found that PITX2 was expressed in Leydig cells, pachytene spermatocytes, round spermatids, and elongating spermatids rather than spermatogonia and leptotene/zygotene spermatocytes. Pitx2b expression significantly increased along with the occurrence of pachytene spermatocytes and round spermatids, while decreased along with the processes of elongated spermatids. In wild male rats with similar testes weight, a significantly suppressed Pitx2b expression occurred with an active meiotic stage in the inhibited testes in autumn and winter, compared with the normally developing testes in spring and summer. These results indicate that Pitx2b expression suppression plays a crucial role in the seasonal inhibition of testis development.

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.

Proliferation and apoptosis processes in the seasonal testicular development of the wild Daurian ground squirrel (Citellus dauricus Brandt, 1844)

The aim of the present study was to elucidate the regulatory role of cell proliferation and apoptosis in testicular development of wild Daurian ground squirrels during the breeding season (April), the non-breeding season (June) and before hibernation (September). Gross mass and hormonal analysis showed that the testis : body mass ratio and plasma testosterone concentration fluctuated seasonally, with a peak in April and lowest values in June. Similarly, spermatogenesis was fully developed in April but suppressed in June and September. Testicular decellularisation and vacuolisation was seen during the transition from the breeding to the non-breeding season. Furthermore, testicular levels of proliferating cell nuclear antigen, cyclin D2 and caspase-3 protein were significantly increased in June and September. Intriguingly, positive terminal deoxyribonucleotidyl transferase-mediated dUTP–digoxigenin nick end-labelling staining and nuclear translocation of caspase-3 in testicular germ cells appeared only during the prehibernation period, whereas accumulation of cyclin D2 in spermatocyte nuclei occurred in September. These findings demonstrate, for the first time, that both cell proliferation and apoptosis are stimulated during the prehibernation period, indicating that a hormonal-regulated balance of testicular germ cell proliferation and apoptosis may play a pivotal role in preparing for testicular recrudescence of wild Daurian ground squirrels.

Sex-specific expression of CTNNB1 in the gonadal morphogenesis of the chicken

BACKGROUND

Beta-catenin (CTNNB1), as a key transcriptional regulator in the WNT signal transduction cascade, plays a pivotal role in multiple biological functions such as embryonic development and homeostasis in adults. Although it has been suggested that CTNNB1 is required for gonad development and maintenance of ovarian function in mice, little is known about the expression and functional role of CTNNB1 in gonadal development and differentiation in the chicken reproductive system.

METHODS

To examine sex-specific, cell-specific and temporal expression of CTNNB1 mRNA and protein during gonadal development to maturation of reproductive organs, we collected left and right gonads apart from mesonephric kidney of chicken embryos on embryonic day (E) 6, E9, E14, E18, as well as testes, oviduct and ovaries from 12-week-old and adult chickens and performed quantitative PCR, in situ hybridization, and immunohistochemical analyses. In addition, localization of Sertoli cell markers such as anti-Müllerian hormone (AMH), estrogen receptor alpha (ESR1), cyclin D1 (CCND1) and N-cadherin (CDH2) during testicular development was evaluated.

RESULTS

Results of the present study showed that CTNNB1 mRNA and protein are expressed predominantly in the seminiferous cords on E6 to E14 in the male embryonic gonad, and are mainly localized to the medullary region of female embryonic gonads from E6 to E9. In addition, CTNNB1 mRNA and protein are abundant in the Sertoli cells in the testes and expressed predominantly in luminal epithelial cells of the oviduct, but not in the ovaries from 12-week-old and adult chickens. Concomitant with CTNNB1, AMH, ESR1, CCND1 and CDH2 were detected predominantly in the seminiferous cord of the medullary region of male gonads at E9 (after sex determination) and then maintained or decreased until hatching. Interestingly, AMH, ESR1, CCND1 and CDH2 were located in seminiferous tubules of the testes from 12-weeks-old chickens and ESR1, CCND1 and CDH2 were expressed predominantly in the Sertoli cells within seminiferous tubules of adult testes.

CONCLUSIONS

Collectively, these results revealed that CTNNB1 is present in gonads of both sexes during embryonic development and it may play essential roles in differentiation of Sertoli cells during formation of seminiferous tubules during development of the testes.

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.

Segment- and cell-specific expression of D-type cyclins in the postnatal mouse epididymis

Sperm transport, maturation and storage are the essential functions of the epididymis. The epididymis in the mouse is structurally characterized by regional and segmental organization including caput, corpus and cauda epididymis that are comprised of 10 segments. Although several growth factor signaling pathways have been discovered in the epididymis, how these converge onto the cell cycle components is unknown. To begin to elucidate the growth factor control of cell cycle events in the epididymis, we analyzed the expression of D-type cyclins at different postnatal ages. At 7d, cyclin D1 was mainly expressed in the cauda epithelium, by 14d its expression occurred in the epithelium of caput, corpus and cauda that persisted up to 21d. By 42d, cyclin D1 was mostly detectable in the principal cells of the caput and corpus (segments 1-7) but not in the cauda epididymis. Expression of cyclin D2, unlike that of cyclin D1, was evident only at 42d but not earlier, and was mostly confined to corpus and cauda epithelium. In contrast to both cyclins D1 and D2, cyclin D3 was expressed primarily in the interstitium at 7d and by 21d its expression was localized to the epithelium of the corpus and cauda epididymis. By 42d, expression of cyclin D3 peaked in segments 6-10 and confined to basal and principal cells of the corpus and apical cells of the cauda epithelium. Ki67 immunoreactivity confirmed absence of cell proliferation despite continued expression of D-type cyclins in the adult epididymis. Collectively, on the basis of our immunophenotyping and protein expression data, we conclude that the D-type cyclins are expressed in a development-, segment-, and cell-specific manner in the postnatal mouse epididymis.

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.

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.

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.

Insulin-Like Growth Factor 1 Is Expressed in Mouse Developing Testis and Regulates Somatic Cell Proliferation

Testicular development occurs prenatally in mammals. The developmental underlying mechanism is only partially understood. The aim of the present investigation was to study the expression of the gene coding for insulin-like growth factor 1 ( Igf-1) and Igf-1 type 1 receptor ( Igf-1r) and their respective proteins in mouse Sertoli and Leydig cells at gestation day 12 (E12)–E18. Moreover, we sought to determine the effect of IGF-1 on the proliferation of both cell types and to establish the signal transduction mechanism involved in the IGF-1 pathway. Transcripts for the Igf-1 and Igf-1r genes were found in Sertoli and Leydig cells from E12–E18. Highest IGF-1 and IGF-Ir protein expression levels were found in both cell types at E18. Exogenous IGF-1 administration increased Sertoli and Leydig cell proliferation at E14–E18 in vitro. Inhibition of the pathway mitogen-activated extracellular signal-regulated protein kinase (MEK) 1/2 with UO126 diminished the proliferation of the Sertoli and Leydig cells in vitro. We propose that IGF-1 and IGF-1r regulate Sertoli and Leydig cell proliferation through the MEK/extracellular-signal-regulated kinase (ERK) 1/2 signal transduction pathway, leading to development and growth of the mouse embryonic testis.

GSK3 beta mediates suppression of cyclin D2 expression by tumor suppressor PTEN

PTEN, encoding a lipid phosphatase, is a tumor suppressor gene and is mutated in various types of cancers. It is reported to regulate G1 to S phase transition of the cell cycle by influencing the expression, protein stability and subcellular location of cyclin D1. Here, we provide evidence that PTEN modulates the transcription and protein stability of cyclin D2. Targeted deletion of Pten in mouse embryonic fibroblasts (MEFs) endowed cells with greater potential to overcome G1 arrest than wild-type MEFs and led to the elevated expression of cyclin D2, which was suppressed by the introduction of PTEN. We further defined a pathway involving GSK3beta and beta-catenin/TCF in PTEN-mediated suppression of cyclin D2 transcription. LiCl, an inhibitor of GSK3beta, abolished inhibitory effect of PTEN on cyclin D2 expression, and TCF members could directly bind to the promoter of cyclin D2 and regulate its transcription in a CREB-dependent manner. Our results indicate that the downregulation of cyclin D2 expression by PTEN is mediated by the GSK3beta/beta-catenin/TCF pathway in cooperation with CREB, and suggest a convergence from the PI-3 kinase/PTEN pathway and the Wnt pathway in modulation of cyclin D2 expression.

Expression profiling of purified male germ cells: stage-specific expression patterns related to meiosis and postmeiotic development

Gene expression profiling was performed using the National Institute on Aging 15,000-cDNA microarray to reveal the differential expression pattern of 160 genes between meiotic pachytene spermatocytes and postmeiotic round spermatids of the mouse. Our results indicate that more genes are expressed in spermatids than in spermatocytes. Genes participating in cell cycle regulation and chromatin structure and dynamics are preferentially expressed in spermatocytes, while genes for protein turnover, signal transduction, energy metabolism, and intracellular transport are prevalent in spermatids. This suggests that a switch of functional requirement occurs when meiotic germ cells differentiate into haploid spermatids. Concordant expression patterns were obtained when quantitative real-time polymerase chain reaction was performed to verify the microarray data. Interestingly, the majority of the differentially expressed genes were underrepresented in mitotic type A spermatogonia, and they were preferentially expressed in the testis. Our results suggest that an even higher proportion of the mouse genome is devoted to male gamete development from meiosis than was previously estimated. We also provide evidence that underscores the advantage of using purified germ cells over whole testes in profiling spermatogenic gene expression to identify transcripts that demonstrate stage-specific expression patterns.

Androgen Regulation of Stage-Dependent Cyclin D2 Expression in Sertoli Cells Suggests a Role in Modulating Androgen Action on Spermatogenesis1

Regulation of spermatogenesis involves stage-dependent androgen action on Sertoli cells, but the pathways involved are unclear. We assessed if cyclin D2 could play a role. In rats, Sertoli cell nuclear, stage-dependent immunoexpression of cyclin D2 switched on after Day 10 and persisted through Day 35, but disappeared by adulthood. However, ethane dimethane sulfonate (EDS)-induced testosterone withdrawal in adult rats for 6 days induced stage-dependent cyclin D2 immunoexpression in Sertoli cells, with highest expression at stages IX-XII and nondetectable at stages VI-VIII (opposite that for androgen receptor [AR] immunoexpression). In EDS-treated rats, a single injection of testosterone but not of estrogen reversed this change in 4 h, and testosterone administration from the time of EDS treatment prevented expression of cyclin D2 in Sertoli cells. The EDS-induced changes in cyclin D2 immunoexpression were matched by changes in expression of Ccnd2 (cyclin D2) mRNA in isolated stage-dissected tubules. Treatment of adult rats with flutamide induced stage-dependent cyclin D2 immunoexpression in Sertoli cells within 18 h, and confocal microscopy revealed that immunoexpression of AR and cyclin D2 were mutually exclusive within individual seminiferous tubules in these animals. Sertoli cell-selective ablation of the AR in mice using Cre/loxP technology also resulted in stage-dependent Sertoli cell cyclin D2 immunoexpression. Downstream from cyclin D2 action is retinoblastoma 1 (RB1), a tumor suppressor protein, immunoexpression of which paralleled stage-dependent AR expression in Sertoli cells; RB1 stage specificity disappeared after EDS treatment. These results point to a non-cell cycle role for cyclin D2 and RB1 in mature Sertoli cells in the stage-dependent mechanisms regulated by AR expression and androgen action.

Expression patterns of mitotic and meiotic cell cycle regulators in testicular cancer and development

Mitotic and meiotic cell cycle regulation is essential for normal development and tumor prevention. The underlying molecular mechanisms are not completely characterized. The aim of our analysis was to derive a global expression map of cell cycle regulators in mitosis and meiosis. First, the expression of cyclins, CDKs and CDK inhibitors was determined during postnatal testis maturation in mice using microarrays and quantitative RT-PCR. The abundance of cyclins A1, B2, K, M4, CDK2, all CDKLs, CDKN2c, CDKN2d and INCA1 increased during testis maturation. In contrast, cyclins A2, B1, D2, G1, G2, CDK1, CDK4 and CDK2AP1 showed a maturation-associated decrease. Gene expression profiles of isolated germ cells and testicular somatic cells confirmed these results. Second, we determined cyclin expression patterns in human normal and malignant testis samples (n = 36) modeling the reciprocal difference between meiosis and mitosis. Testicular tumors strictly expressed cell cycle regulators identified in mitotically dividing germ cells. Expression of several transcripts was histology-specific in testicular tumors, providing novel molecular markers and potential therapeutic targets. Taken together, our data provide a comprehensive expression map of cell cycle regulators at the switch between mitosis and meiosis in testis development and in cancerogenesis.

Hypomorphic mutation in an essential cell-cycle kinase causes growth retardation and impaired spermatogenesis

Cdc7 kinase is essential for initiation of DNA replication. Cdc7(-/-) mouse embryonic stem (ES) cells are non-viable but their growth can be rescued by an ectopically expressed transgene (Cdc7(-/-)tg). Here we report that, despite the normal growth capability of Cdc7(-/-)tg ES cells, the mice with the identical genetic background exhibit growth retardation. Concomi tantly, Cdc7(-/-)tg embryonic fibroblasts (MEFs) display delayed S phase entry and slow S phase progression. Furthermore, spermatogenesis of Cdc7(-/-)tg mice is disrupted prior to pachytene stage of meiotic prophase I. The impairment in spermatogenesis correlates with the extremely low level of Cdc7 protein in testes, and is rescued by introducing an additional allele of transgene, which results in increase of Cdc7 expression. The increased level of Cdc7 also recovers the growth of Cdc7(-/-)tg MEFs and mice, indicating that the developmental abnormalities of Cdc7(-/-)tg mice are due to insufficiency of Cdc7 protein. Our results indicate the requirement of a critical level of a cell-cycle regulator for mouse development and provide genetic evidence that Cdc7 plays essential roles in meiotic processes in mammals.

Development of mice expressing a single D-type cyclin

D-cyclins (cyclins D1, D2, and D3) are components of the core cell cycle machinery. To directly test the ability of each D-cyclin to drive development of various lineages, we generated mice expressing only cyclin D1, or only cyclin D2, or only cyclin D3. We found that these "single-cyclin" embryos develop normally until late gestation. Our analyses revealed that in single-cyclin embryos, the tissue-specific expression pattern of D-cyclins was lost. Instead, mutant embryos ubiquitously expressed the remaining D-cyclin. These findings suggest that the functions of the three D-cyclins are largely exchangeable at this stage. Later in life, single-cyclin mice displayed focused abnormalities, resulting in premature mortality. "Cyclin D1-only" mice developed severe megaloblastic anemia, "cyclin D2-only" mice presented neurological abnormalities, and "cyclin D3-only" mice lacked normal cerebella. Analyses of the affected tissues revealed that these compartments failed to sufficiently up-regulate the remaining, intact D-cyclin. In particular, we found that in cerebellar granule neuron precursors, the N-myc transcription factor communicates with the cell cycle machinery via cyclins D1 and D2, but not D3, explaining the inability of D3-only mice to up-regulate cyclin D3 in this compartment. Hence, the requirement for a particular cyclin in a given tissue is likely caused by specific transcription factors, rather than by unique properties of cyclins.

Expression of stress response genes in germ cells during spermatogenesis

During germ cell development different spermatogenic cell types show remarkable variation in their susceptibility to stressful stimuli. Various cellular mechanisms are triggered in germ cells after exposure to stress, but the expression of only a few of the genes involved in such pathways has been studied during spermatogenesis. In the present study we determined the expression profiles of 216 stress response genes in isolated rat germ cells (pachytene spermatocytes, and round and elongating spermatids) using cDNA atlas arrays. Of the 216 genes studied, 86 were detected in pachytene spermatocytes, 82 in round spermatids, and 52 in elongating spermatids. Fifty percent (48) of the total number of genes detected during spermatogenesis were detected in all three cell types while nearly 25% (25) were expressed exclusively in pachytene spermatocytes and round spermatids; some cell specific transcripts were observed also. The use of the K means clustering method allowed us to group genes by their pattern of expression during spermatogenesis; five specific expression profiles were obtained and analyzed. To determine how stress response genes are regulated throughout spermatogenesis, we examined the expression of genes involved in stress response mechanisms such as heat shock proteins-chaperones, DNA repair, and oxidative stress. Genes belonging to these families were differentially expressed during germ cell development. We suggest that the differential expression of stress response genes during spermatogenesis contributes to the selectivity of the susceptibility of germ cells to stress.

Control of spermatogenesis in mice by the cyclin D-dependent kinase inhibitors p18(Ink4c) and p19(Ink4d)

Male mice lacking both the Ink4c and Ink4d genes, which encode two inhibitors of D-type cyclin-dependent kinases (Cdks), are infertile, whereas female fecundity is unaffected. Both p18(Ink4c) and p19(Ink4d) are expressed in the seminiferous tubules of postnatal wild-type mice, being largely confined to postmitotic spermatocytes undergoing meiosis. Their combined loss is associated with the delayed exit of spermatogonia from the mitotic cell cycle, leading to the retarded appearance of meiotic cells that do not properly differentiate and instead undergo apoptosis at an increased frequency. As a result, mice lacking both Ink4c and Ink4d produce few mature sperm, and the residual spermatozoa have reduced motility and decreased viability. Whether or not Ink4d is present, animals lacking Ink4c develop hyperplasia of interstitial testicular Leydig cells, which produce reduced levels of testosterone. The anterior pituitary of fertile mice lacking Ink4c or infertile mice doubly deficient for Ink4c and Ink4d produces normal levels of luteinizing hormone (LH). Therefore, the failure of Leydig cells to produce testosterone is not secondary to defects in LH production, and reduced testosterone levels do not account for infertility in the doubly deficient strain. By contrast, Ink4d-null or double-null mice produce elevated levels of follicle-stimulating hormone (FSH). Because Ink4d-null mice are fertile, increased FSH production by the anterior pituitary is also unlikely to contribute to the sterility observed in Ink4c/Ink4d double-null males. Our data indicate that p18(Ink4c) and p19(Ink4d) are essential for male fertility. These two Cdk inhibitors collaborate in regulating spermatogenesis, helping to ensure mitotic exit and the normal meiotic maturation of spermatocytes.

Meiotic expression of the cyclin H/Cdk7 complex in male germ cells of the mouse

Cell division requires that cyclin-dependent kinases (Cdks) be activated by phosphorylation. In mitotic cells, this is accomplished by the Cdk-activating-kinase (CAK), which is a complex of cyclin H and Cdk7. There are currently no data on the role of CAK in meiotic cells. Previously, we have shown that cyclin A1 is meiosis-specific and forms an active kinase with Cdk2. Because cyclin A1 is required for meiosis, and its associated kinase must be phosphorylated (activated), we propose that cyclin H/Cdk7 function to activate cyclin A1/Cdk2 in meiotic cells. Here, we show that cyclin H and Cdk7 are present during meiosis. Using reverse transcription-polymerase chain reaction and in situ hybridization, we show that the mRNAs encoding cyclin H and Cdk7 are abundant in spermatocytes. Immunohistochemistry localized cyclin H and Cdk7 to the nucleus of spermatocytes in stages IV to XII of the spermatogenic cycle, overlapping the same stages that express cyclin A1-associated kinases. Finally, immunoprecipitation and histone H1-kinase assays of cyclin H and Cdk7 from testicular extracts show that these proteins interact to form an active kinase. We conclude that cyclin H/Cdk7 complexes are present and during meiosis, form active complexes in testicular cells and are strong candidates for the activating kinase for cyclin A1-associated kinase.

Involvement of the D-type cyclins in germ cell proliferation and differentiation in the mouse

Using immunohistochemistry, the expression of the D-type cyclin proteins was studied in the developing and adult mouse testis. Both during testicular development and in adult testis, cyclin D(1) is expressed only in proliferating gonocytes and spermatogonia, indicating a role for cyclin D(1) in spermatogonial proliferation, in particular during the G(1)/S phase transition. Cyclin D(2) is first expressed at the start of spermatogenesis when gonocytes produce A(1) spermatogonia. In the adult testis, cyclin D(2) is expressed in spermatogonia around stage VIII of the seminiferous epithelium when A(al) spermatogonia differentiate into A(1) spermatogonia and also in spermatocytes and spermatids. To further elucidate the role of cyclin D(2) during spermatogenesis, cyclin D(2) expression was studied in vitamin A-deficient testis. Cyclin D(2) was not expressed in the undifferentiated A spermatogonia in vitamin A-deficient testis but was strongly induced in these cells after the induction of differentiation of most of these cells into A(1) spermatogonia by administration of retinoic acid. Overall, cyclin D(2) seems to play a role at the crucial differentiation step of undifferentiated spermatogonia into A(1) spermatogonia. Cyclin D(3) is expressed in both proliferating and quiescent gonocytes during testis development. Cyclin D(3) expression was found in terminally differentiated Sertoli cells, in Leydig cells, and in spermatogonia in adult testis. Hence, although cyclin D(3) may control G(1)/S transition in spermatogonia, it probably has a different role in Sertoli and Leydig cells. In conclusion, the three D-type cyclins are differentially expressed during spermatogenesis. In spermatogonia, cyclins D(1) and D(3) seem to be involved in cell cycle regulation, whereas cyclin D(2) likely has a role in spermatogonial differentiation.

A single dose of Di-(2-ethylhexyl) phthalate in neonatal rats alters gonocytes, reduces sertoli cell proliferation, and decreases cyclin D2 expression

In this study, we explored the impact on both Sertoli cells and gonocytes of a single, relatively low dose of di-(2-ethylhexyl) phthalate (DEHP; 20-500 mg/kg) administered in vivo to 3-day-old rat pups. In parallel, we assessed the potential for two immediate metabolites of DEHP to produce similar testicular changes and began to explore the possible mechanisms involved. Morphological examination revealed the presence of many abnormally large, multi-nucleated germ cells by 24 h posttreatment with DEHP and with its metabolite, mono-ethylhexyl phthalate (MEHP), but not with another metabolite, 2-ethylhexanol (2-EH; all at 1.28 mmol/kg) or with vehicle alone. These cells persisted through 48 h posttreatment, the longest time point examined in our study. We also assessed the rate of Sertoli cell proliferation in pups at intervals after dosage with either chemical or vehicle by administering bromodeoxy uridine (BrdU) 3 h before euthanasia. By 24 h after treatment with DEHP or MEHP, but not 2-EH or vehicle, the number of BrdU-labeled Sertoli cells was obviously diminished in testicular sections. Quantitation of DEHP-treated pups and controls indicates that a dose-response relationship exists between chemical treatment and labeling index (LI) of Sertoli cells, with a LI at the highest DEHP dose tested that was only 20% of that in controls. In addition, when we examined the time course of the effect of an intermediate dose of DEHP, we found that there the LI of Sertoli cells rebounds by 48 h after dosage, when we found the rate of proliferation in treated pups to be significantly higher than in controls. We also explored the potential mechanism involved in the response to DEHP and found serum levels of FSH to be unaffected by the chemical. In addition, study of cell cycle-related proteins including p27kip1 and cyclins D1, D2, and D3 with Western and Northern analysis indicated that cyclin D2 mRNA is specifically down-regulated by DEHP in a dose-dependent manner, and this decrease is manifest as a small, transient but reproducible reduction in the amount of cyclin D2 protein detectable in samples from treated pups compared to controls. Our findings characterize the changes in neonatal Sertoli cells and gonocytes that follow in vivo to low levels of DEHP and its metabolite, MEHP, as well as providing new information on the underlying mechanism and highlighting the extreme sensitivity of the neonatal testis to injury by this toxicant.

Developmentally regulated expression of cyclin D3 and its potential in vivo interacting proteins during murine gametogenesis

To begin to assess the function of the cell cycle regulator cyclin D3 during gametogenesis, the present study examined its expression, interacting partners, and associated kinase activity in the murine testis and ovary. In the early stages of postnatal testicular development, cyclin D3 protein was detected in spermatogonia and Leydig cells. In the adult testis, cyclin D3 was also expressed in terminally differentiating spermatids. In the embryonic ovary, detection of cyclin D3 was limited to somatic cells. In the postnatal ovary, its localization was predominantly in the nuclei of oocytes in primordial and small follicles, a localization that diminished with oocyte growth. Cdk4 and p27 were expressed in a similar subset of testicular and ovarian cells, suggesting that they may regulate cyclin D3 function during testicular and ovarian development in a cell type-specific manner. Cyclin D3-associated kinase activity was detected in immature, but not adult, testes and ovaries. These observations suggest unique roles for cyclin D3 in the control of cell division and differentiation in the germ line and the differential regulation of mitotic and meiotic cell cycles during male and female gametogenesis.

The self-renewing mechanism of stem cells in the germline

Germline stem cells (GSCs) are a self-renewing population of germ cells that serve as the source of gametes in diverse organisms. Current research suggests that the self-renewing division of GSCs is controlled both by somatic signaling and by intracellular mechanisms such as differential gene expression, asymmetric cytoskeletal organization, and the cell cycle machinery. These findings provide a framework for the further study of GSCs and stem cell renewal in general.

Cyclin D3 expression in normal, reactive and neoplastic tissues

Cyclin D3 immunohistochemical expression was investigated in normal, reactive, and neoplastic human embryonal and adult tissues. In the fetus, cyclin D3 was expressed in selected developmental phases of a limited number of cell systems. In normal adult tissues, cyclin D3 showed two patterns of distribution: in lymphoid tissues it was expressed in proliferative compartments, while in most other tissues it was expressed by terminally differentiated/quiescent cells. This dual role in proliferation and differentiation was partially conserved in neoplasms. In non-Hodgkin lymphomas, cyclin D3 immunolabelling was correlated with proliferative activity and progression; a significant exception was seen in cyclin D1-positive mantle cell lymphomas, which were cyclin D-negative. Benign endocrine tumours were frequently strongly cyclin D3-positive, while high-grade (small cell) neuroendocrine carcinomas were always negative. In most other epithelial neoplasms, cyclin D3 immunostaining was heterogeneous. In breast carcinomas, no relationship was seen between ER status and MIB1 labelling; cyclins D3 and D1 were frequently expressed in the same tumour, while occasional tumours showed an inverse quantitative relationship between cyclins D1 and D3, and rare tumours were negative for both. In soft tissue neoplasms, cyclin D3 was consistently expressed in some tumours, such as stromal tumours of the gastrointestinal tract and embryonal rhabdomyosarcomas. Our data suggest that cyclin D3 has a dual role in proliferation and differentiation in normal tissues and in some neoplastic conditions; that the cyclin D3 expression pattern is different from cyclin D1, suggesting non-redundant functions; that cyclin D3 expression is strong in endocrine cells secreting steroid hormones, and in their neoplastic counterparts; and that cyclin D3 deregulation may be of pathogenetic relevance in lymphomagenesis and could be diagnostically useful.

The identification and characterization of expression of Pftaire-1, a novel Cdk family member, suggest its function in the mouse testis and nervous system

We have isolated a murine cDNA encoding for a novel putative Cdk-related protein kinase, which has been named Pftaire-1, by screening a testis cDNA library for new serine/threonine kinases. Pftaire-1 showed 50% and 49% amino acid identity with Cdk5 and Pctaire-3, respectively, and contains the eleven subdomains characteristic of the protein kinases. By northern blot analysis we detected two transcripts of approximately 5.5 and 4.9 kb in size. These transcripts were expressed at low level in all murine tissues tested, except in the brain, testis and embryo, where high expression was detected. Cellular localization of the mRNAs by in situ hybridization analysis shows that Pftaire-1 is expressed in late pachytene spermatocytes in the testis and in post mitotic neuronal cells both in the brain and the embryo, suggesting a role of Pftaire-1 both in the process of meiosis as well as neuron differentiation and/or function.

G1-phase regulators, cyclin D1, cyclin D2, and cyclin D3: up-regulation at gastrulation and dynamic expression during neurulation

Gastrulation in rodents is associated with an increase in the rate of growth and with the start of differentiation within the embryo proper. In an effort to understand the role played by the cell cycle control in these processes, expression of cyclin D1, D2, and D3--three major positive regulators of the G1/S transition--has been investigated by in situ hybrization and RT-PCR. Cyclin D1 and D2 transcripts are first detected in the epiblast at gastrulation, when a proliferative burst occurs, and subsequently in its differentiated derivatives within the embryo proper, indicating that activation of their expression takes place prior to the differentiation of epiblast progenitors. In contrast, cyclin D3 transcript is undetectable in the epiblast itself and its expression is activated exclusively in extraembryonic tissues of both epiblast and trophoblast origin. During neurulation, expression of each cyclin D RNA is dynamically regulated along the anterior-posterior axis. In the hindbrain, cyclin D1 and D2 show distinct segment-specific restricted expression and this pattern is conserved between mouse and chick. These results strongly suggest that D-type cyclins act as developmental regulators.

A requirement for cyclin D3-cyclin-dependent kinase (cdk)-4 assembly in the cyclic adenosine monophosphate-dependent proliferation of thyrocytes

In different systems, cyclic adenosine monophosphate (cAMP) either blocks or promotes cell cycle progression in mid to late G1 phase. Dog thyroid epithelial cells in primary culture constitute a model of positive control of DNA synthesis initiation and G0-S prereplicative phase progression by cAMP as a second messenger for thyrotropin (TSH). The cAMP-dependent mitogenic pathway is unique as it is independent of mitogen-activated protein kinase activation and differs from growth factor-dependent pathways at the level of the expression of several protooncogenes/transcription factors. This study examined the involvement of D-type G1 cyclins and their associated cyclin-dependent kinase (cdk4) in the cAMP-dependent G1 phase progression of dog thyroid cells. Unlike epidermal growth factor (EGF)+serum and other cAMP-independent mitogens, TSH did not induce the accumulation of cyclins D1 and D2 and partially inhibited the basal expression of the most abundant cyclin D3. However, TSH stimulation enhanced the nuclear detection of cyclin D3. This effect correlated with G1 and S phase progression. It was found to reflect both the unmasking of an epitope of cyclin D3 close to its domain of interaction with cdk4, and the nuclear translocation of cyclin D3. TSH and EGF+serum also induced a previously undescribed nuclear translocation of cdk4, the assembly of precipitable cyclin D3-cdk4 complexes, and the Rb kinase activity of these complexes. Previously, cdk4 activity was found to be required in the cAMP-dependent mitogenic pathway of dog thyrocytes, as in growth factor pathways. Here, microinjections of a cyclin D3 antibody showed that cyclin D3 is essential in the TSH/ cAMP-dependent mitogenesis, but not in the pathway of growth factors that induce cyclins D1 and D2. The present study (a) provides the first example in a normal cell of a stimulation of G1 phase progression occurring independently of an enhanced accumulation of cyclins D, (b) identifies the activation of cyclin D3 and cdk4 through their enhanced assembly and/or nuclear translocation, as first convergence steps of the parallel cAMP-dependent and growth factor mitogenic pathways, and (c) strongly suggests that this new mechanism is essential in the cAMP-dependent mitogenesis, which provides the first direct demonstration of the requirement for cyclin D3 in a G1 phase progression.

The tao of stem cells in the germline

Germline stem cells (GSCs) are the self-renewing population of germ cells that serve as the source for gametogenesis. GSCs exist in diverse forms, from those that undergo strict self-renewing asymmetric divisions in Drosophila to those that maintain their population by balancing between mitosis and differentiation in Caenorhabditis elegans. Most vertebrate spermatogonial GSCs appear to adopt an intermediate strategy. In most animals, GSCs are established during preadult gonadogenesis following the proliferation and migration of embryonic primordial germ cells. GSCs produce numerous gametes throughout the sexually active period of adult life. The establishment and self-renewing division of GSCs are controlled by extracellular signals such as hormones from the hypothalamic-pituitary axis and local interactions between GSCs and their neighboring cells. These extracellular signals may then influence differential gene expression, cell cycle machinery, and cytoskeletal organization of GSCs for their formation and/or divisional asymmetry. In addition, the GSC mechanism is related to that for germline and sex determination. Current knowledge has provided a solid framework for further study of GSCs and stem cells in general.

HSP70-2 is required for CDC2 kinase activity in meiosis I of mouse spermatocytes

Cyclin B-dependent CDC2 kinase activity has a key role in triggering the G2/M-phase transition during the mitotic and meiotic cell cycles. The Hsp70-2 gene is expressed only in spermatogenic cells at a significant level. In Hsp70-2 gene knock-out (Hsp70-2(−/−)) mice, primary spermatocytes fail to complete meiosis I, suggesting a link between HSP70-2 heat-shock protein and CDC2 kinase activity during this phase of spermatogenesis. Members of the HSP70 protein family are molecular chaperones that mediate protein de novo folding, translocation and multimer assembly. This study used immunoprecipitation-coupled western blot and in vitro reconstitution experiments to show that HSP70-2 interacts with CDC2 in the mouse testis, appears to be a molecular chaperone for CDC2, and is required for CDC2/cyclin B1 complex formation. Previous studies reported that most CDC2 kinase activity in the mouse testis is present in pachytene spermatocytes. Although CDC2 kinase activity for histone H1 was present in the testis of wild-type mice, it was nearly absent from the testis of Hsp70-2(−/−) mice, probably due to defective CDC2/cyclin B1 complex formation. Furthermore, addition of HSP70-2 to freshly prepared extracts of testis from Hsp70-2(−/−) mice not only restored CDC2/cyclin B1 complex formation but also reconstituted CDC2 kinase activity in vitro. It appears that one cause of failure to complete meiosis I during spermatogenesis in Hsp70-2(−/−) mice is disruption of CDC2/cyclin B1 assembly in pachytene spermatocytes, thereby preventing development of the CDC2 kinase activity required to trigger G2/M-phase transition. These studies provide novel in vivo evidence for a link between an HSP70 molecular chaperone and CDC2 kinase activity essential for the meiotic cell cycle in spermatogenesis.

Cytoplasmic localization of cyclin D3 in seminiferous tubules during testicular development

Using a newly developed polyclonal antibody against murine cyclin D3, we have found that protein levels of cyclin D3 were highly detectable only in thymus and testis in rats. Since testis offer unique opportunities to examine the cell cycle in vivo, we examined the temporal and spatial expression of cyclin D3 and the DNA synthesis indicator, proliferating cell nuclear antigen (PCNA), in the rat testis during development. The protein levels of cyclin D3 protein in testis from 7 days to 3 months old were almost constant and then decreased gradually thereafter. The protein levels of cyclin D1 and PCNA were high in the testis of 7- and 14-day-old rats and decreased during testicular development. In the seminiferous tubules of 7-day-old newborns, cyclin D3 was surprisingly located in cytoplasm of stem cells that had bigger nuclei than the nuclei of surrounding cells. Interestingly, cyclin D3 immunopositive cells did not immunostain with PCNA in nuclei. In the adult testis, anti-cyclin D3 antibody strongly stained the cytoplasm of early stage primary spermatocytes, lightly stained pachytene spermatocytes, but did not stain elongated spermatids. There was no detectable cyclin D3 in Sertoli cells, interstitial cells, or fibroblasts within seminiferous tubules, or in blood vessels within the interstitial matrix. The known cyclin D3 partner, cyclin dependent kinase 4, was located mainly in nuclei of spermatogonia and in early stage primary spermatocytes. Strong PCNA immunopositive staining was located in the nuclei of spermatogonia in adult testis. These results indicate that cyclin D3 is detectable in meiotically active male germ cells (PCNA-negative cells), but is conspicuously absent from mitotically active spermatogonia (PCNA-positive cells). Moreover, in contrast to in vitro reports, cyclin D3 is not located in the nucleus, but rather in the cytoplasm of male germ cells in vivo. Taken together, the presence of cyclin D3 in spermatocytes and its location in the cytoplasm lead us to speculate that cyclin D3 may have functions in male germ cells other than mitosis.

Developmental control of cell cycle regulators: a fly's perspective

During early development in many species, maternally supplied gene products permit the cell cycle to run at maximum velocity, subdividing the fertilized egg into smaller and smaller cells. As development proceeds, zygotic controls are activated that first limit divisions to defined spatial and temporal domains, coordinating them with morphogenesis, and then halt proliferation altogether, to allow cell differentiation. Analysis of the regulation of cyclin-dependent kinases (Cdks) in Drosophila has provided insights into how this embryonic program of cell proliferation is controlled at the molecular level and how it is linked to developmental cues. Recent studies have also begun to reveal how cell proliferation is controlled during the second phase of Drosophila development, which occurs in imaginal tissues. In contrast to their embryonic progenitors, imaginal cells proliferate with a cycle that requires cell growth and is linked to patterning processes controlled by secreted cell signaling molecules. The functions of these signaling molecules appear to be nearly as conserved between vertebrates and invertebrates as the cell cycle control apparatus itself, suggesting that the mechanisms that coordinate growth, patterning, and cell proliferation in developing tissues have ancient origins.

The Pem homeobox gene. Androgen-dependent and -independent promoters and tissue-specific alternative RNA splicing

The Pem gene encodes an atypical homeodomain protein, distantly related to Prd/Pax family members, that we demonstrate is regulated in a complex transcriptional and post-transcriptional manner. We show that the rat Pem genomic structure includes three 5'-untranslated (5'-UT) exons and four coding exons, three of which encode the homeodomain. Several alternatively spliced transcripts were identified, including one that skips an internal coding exon, enabling this mRNA to express a novel form of the Pem protein. Other alternatively spliced mRNAs were characterized that possess different 5'-UT regions, including a muscle-specific transcript. The different 5'-UT termini present in Pem transcripts conferred different levels of translatability in vitro. Two promoters containing multiple transcription initiation sites were identified: a distal promoter (Pd) in the first 5'-UT exon and a proximal promoter (Pp) located in the "intron" upstream of the first coding exon. The Pd was active in placenta, ovary, tumor cell lines, and to a lesser extent in skeletal muscle. In contrast, transcripts from the Pp were only detectable in testis and epididymis and were only expressed in epididymis in the presence of testosterone. To our knowledge no transcription factors have previously been identified that exhibit androgen-dependent expression in the epididymis.

Cdk family genes are expressed not only in dividing but also in terminally differentiated mouse germ cells, suggesting their possible function during both cell division and differentiation

The roles of the cyclin dependent kinase (Cdk) family in murine germ cell development have been examined by studying the expression of five Cdk family genes (Cdc2, Cdk2, Cdk4, Pctaire-1, and Pctaire-3) in mouse reproductive organs. Northern blot and in situ hybridization analyses revealed distinctive expression patterns of these genes with striking cellular, lineage, and developmental stage specificity. We observed Cdk expression in cell types with proliferative activity: Cdc2 and Cdk2 expression in premeiotic spermatocytes in the testis, and Cdc2, Cdk2, and Cdk4 expression in granulosa cells of ovarian follicles. Cdc2 transcripts were most abundant in late pachytene to diplotene spermatocytes, soon to undergo meiosis. Surprisingly, we also observed expression of Cdk family genes in non-proliferating cell types. All five Cdk family genes examined were expressed in Sertoli cells of the adult testis, which are no longer mitotically active. With regard to Pctaire-1 and Pctaire-3, the highest levels of expression were observed in postmeiotic spermatids. Immunoblot analysis also revealed the presence of high levels of Pctaire-1 in postmeiotic germ cells. These results suggest that Cdk family kinases may exhibit various functions in germinal and somatic cells during gametogenesis, not only in the cell cycle but also in other regulatory processes, including differentiation.

Diversification of cell cycle controls in developing embryos

During embryogenesis, a genetic program coordinates cell proliferation with morphogenesis and cell differentiation. Recent studies using Drosophila have shown how, as development proceeds, this program directs different cell types to acquire unique modes of cell cycle regulation. As maternal cell cycle factors are exhausted and replaced by differentially expressed zygotic factors, an increasing repertoire of gene products become potential regulators of the cycle. Cyclin B, Cdc25, and Cyclin E each act as limiting regulators in Drosophila in specific cell types at particular developmental stages. The genes encoding these and many other candidate regulators have been cloned from mice, but their roles in vivo have yet to be understood.