Cyclin A1 is required for meiosis in the male mouse

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

Cell-cycle transition at G2-M is controlled by MPF (M-phase-promoting factor), a complex consisting of the Cdc2 kinase and a B-type cyclin. We have shown that in mice, targeted disruption of an A-type cyclin gene, cyclin A1, results in a block of spermatogenesis prior to the entry into metaphase I. The meiotic arrest is accompanied by a defect in Cdc2 kinase activation at the G2--M transition, raising the possibility that a cyclin A1-dependent process dictates the activation of MPF. Here we show that like Cdc2, the expression of B-type cyclins is retained in cyclin A1-deficient spermatocytes, while their associated kinases are kept at inactive states. Treatment of arrested germ cells with the protein phosphatase type-1 and -2A inhibitor okadaic acid restores the MPF activity and induces entry into M phase and the formation of normally condensed chromosome bivalents, concomitant with hyperphosphorylation of Cdc25 proteins. Conversely, inhibition of tyrosine phosphatases, including Cdc25s, by vanadate suppresses the okadaic acid-induced metaphase induction. The highest levels of Cdc25A and Cdc25C expression and their subcellular localization during meiotic prophase coincide with that of cyclin A1, and when overexpressed in HeLa cells, cyclin A1 coimmunoprecipitates with Cdc25A. Furthermore, the protein kinase complexes consisting of cyclin A1 and either Cdc2 or Cdk2 phosphorylate both Cdc25A and Cdc25C in vitro. These results suggest that in normal meiotic male germ cells, cyclin A1 participates in the regulation of other protein kinases or phosphatases critical for the G2-M transition. In particular, it may be directly involved in the initial amplification of MPF through the activating phosphorylation on Cdc25 phosphatases.

Early development of mouse embryos null mutant for the cyclin A2 gene occurs in the absence of maternally derived cyclin A2 gene products

Progression through the mammalian cell cycle is regulated by the sequential activation and inactivation of the cyclin-dependent kinases. In adult cells, cyclin A2-dependent kinases are required for entry into S and M phases, completion of S phase, and centrosome duplication. However, mouse embryos lacking the cyclin A2 gene nonetheless complete preimplantation development, but die soon after implantation. In this report, we investigated whether a contribution of maternal cyclin A2 mRNA and protein to early embryonic cell cycles might explain these conflicting observations. Our data show that a maternal stock of cyclin A2 mRNA is present in the oocyte and persists after fertilization until the second mitotic cell cycle, when it is degraded to undetectable levels coincident with transcriptional activation of the zygotic genome. A portion of maternally derived cyclin A2 protein is stable during the first mitosis and persists in the cytoplasm, but is completely degraded at the second mitosis. The ability of cyclin A2-null mutants to develop normally from the four-cell to the postimplantation stage in the absence of detectable cyclin A2 gene product indicates therefore that cyclin A2 is dispensable for cellular progression during the preimplantation nongrowth period of mouse embryo development.

Functional compensation by Egr4 in Egr1-dependent luteinizing hormone regulation and Leydig cell steroidogenesis

The Egr family of zinc finger transcription factors, whose members are encoded by Egr1 (NGFI-A), Egr2 (Krox20), Egr3, and Egr4 (NGFI-C) regulate critical genetic programs involved in cellular growth, differentiation, and function. Egr1 regulates luteinizing hormone beta subunit (LHbeta) gene expression in the pituitary gland. Due to decreased levels of LHbeta, female Egr1-deficient mice are anovulatory, have low levels of progesterone, and are infertile. By contrast, male mutant mice show no identifiable defects in spermatogenesis, testosterone synthesis, or fertility. Here, we have shown that serum LH levels in male Egr1-deficient mice are adequate for maintenance of Leydig cell steroidogenesis and fertility because of partial functional redundancy with the closely related transcription factor Egr4. Egr4-Egr1 double mutant male mice had low steady-state levels of serum LH, physiologically low serum levels of testosterone, and atrophy of androgen-dependent organs that were not present in either Egr1- or Egr4-deficient males. In double mutant male mice, atrophic androgen-dependent organs and Leydig cell steroidogenesis were fully restored by administration of exogenous testosterone or human chorionic gonadotropin (an LH receptor agonist), respectively. Moreover, a normal distribution of gonadotropin-releasing hormone-containing neurons and normal innervation of the median eminence in the hypothalamus, as well as decreased levels of LH gene expression in Egr4-Egr1-relative to Egr1-deficient male mice, indicates a defect of LH regulation in pituitary gonadotropes. These results elucidate a novel level of redundancy between Egr4 and Egr1 in regulating LH production in male mice.

Molecular cloning, chromosome mapping and characterization of UBQLN3 a testis-specific gene that contains an ubiquitin-like domain

The sequence of the ubiquitin protein is highly conserved between species and has facilitated the cloning of numerous ubiquitin-like proteins. In the present study, we report the cloning of the cDNA for human ubiquilin 3 (UBQLN3). The deduced amino acid sequence of UBQLN3 contains a UBQ domain (ubiquitin-like) in the amino terminus as well as two highly conserved domains found in several recently cloned ubiquitin-like proteins. One of these domains, termed the NP domain, is a highly conserved 93 amino acid region present in UBQLN3 and several ubiquitin-like proteins. The last conserved domain is the UBA domain (ubiquitin-associated) found in a variety of proteins of the ubiquination pathway. The human UBQLN3 gene was mapped to the 11p15 region of chromosome 11. Northern blot analysis of multiple human and mouse tissues demonstrated UBQLN3 mRNA expression specifically in testis.

Stage-specific nuclear antigen is expressed in rat male germ cells during early meiotic prophase

A germ cell nuclear antigen with approximately 44-kDa molecular weight was identified by a novel monoclonal antibody designated as Mab 2F2 from the library we have accumulated against rat testicular cells. In immature 20-day-old and adult rat testis the recognized antigen was expressed in the nuclei of early meiotic cells from preleptotene to early pachytene spermatocytes exhibiting a stage-specific appearance in the cycle of the seminiferous epithelium. The immunoreactivity was clearly associated with the meiotic chromosomes. The antigen was not detected in the late pachytene spermatocytes and more advanced stages of spermatogenesis. No labeling was observed in spermatogonia and somatic Sertoli and Leydig cells. The pattern of expression of the recognized antigen during early meiotic stages of spermatogenesis but not in mitotically dividing spermatogonia could strengthen its possible role in meiotic division.

Specific expression of soluble adenylyl cyclase in male germ cells

The cAMP signaling pathway is an important mediator of extracellular signals in organisms from prokaryotes to higher eukaryotes. In mammals two types of adenylyl cyclase synthesize cAMP; a ubiquitous family of transmembrane isoforms regulated by G proteins in response to extracellular signals, and a recently isolated soluble enzyme insensitive to heterotrimeric G protein modulation. Using the very sensitive reverse transcription-polymerase chain reaction (RT-PCR), soluble adenylyl cyclase (sAC) expression is detectable in almost all tissues examined; however, Northern analysis and in situ hybridization indicate that high levels of sAC message are unique to male germ cells. Elevated levels of sAC mRNA are first observed in pachytene spermatocytes and expression increases through spermiogenesis. The accumulation of high levels of message in round spermatids suggests sAC protein plays an important role in the generation of cAMP in spermatozoa, implying possible roles in sperm maturation through the epididymis, capacitation, hypermotility, and/or the acrosome reaction.

Methylation of the cyclin A1 promoter correlates with gene silencing in somatic cell lines, while tissue-specific expression of cyclin A1 is methylation independent

Gene expression in mammalian organisms is regulated at multiple levels, including DNA accessibility for transcription factors and chromatin structure. Methylation of CpG dinucleotides is thought to be involved in imprinting and in the pathogenesis of cancer. However, the relevance of methylation for directing tissue-specific gene expression is highly controversial. The cyclin A1 gene is expressed in very few tissues, with high levels restricted to spermatogenesis and leukemic blasts. Here, we show that methylation of the CpG island of the human cyclin A1 promoter was correlated with nonexpression in cell lines, and the methyl-CpG binding protein MeCP2 suppressed transcription from the methylated cyclin A1 promoter. Repression could be relieved by trichostatin A. Silencing of a cyclin A1 promoter-enhanced green fluorescent protein (EGFP) transgene in stable transfected MG63 osteosarcoma cells was also closely associated with de novo promoter methylation. Cyclin A1 could be strongly induced in nonexpressing cell lines by trichostatin A but not by 5-aza-cytidine. The cyclin A1 promoter-EGFP construct directed tissue-specific expression in male germ cells of transgenic mice. Expression in the testes of these mice was independent of promoter methylation, and even strong promoter methylation did not suppress promoter activity. MeCP2 expression was notably absent in EGFP-expressing cells. Transcription from the transgenic cyclin A1 promoter was repressed in most organs outside the testis, even when the promoter was not methylated. These data show the association of methylation with silencing of the cyclin A1 gene in cancer cell lines. However, appropriate tissue-specific repression of the cyclin A1 promoter occurs independently of CpG methylation.

Degradation of cyclin A does not require its phosphorylation by CDC2 and cyclin-dependent kinase 2

Many cyclins are degraded by the ubiquitination/proteasome pathways involving the anaphase-promoting complex and SCF complexes. These degradations are frequently dependent on phosphorylation by cyclin-dependent kinases (CDKs), providing a self-limiting mechanism for CDK activity. Here we present evidence from in vitro and in vivo assay systems that the degradation of human cyclin A can be inhibited by kinase-inactive mutants of CDK2 and CDC2. One obvious interpretation of these results is that like other cyclins, CDK-dependent phosphorylation of the cyclin A may be involved in cyclin A degradation. Our data indicated that CDK2 can phosphorylate cyclin A on Ser-154. Site-directed mutagenesis of Ser-154 abolished the phosphorylation by recombinant CDK2 in vitro and the majority of cyclin A phosphorylation in the cell. Activation of CDK2 and binding to SKP2 or p27(KIP1) were not affected by the phosphorylation of Ser-154. Surprising, in marked contrast to cyclin E, where phosphorylation of Thr-380 by CDK2 is required for proteolysis, degradation of cyclin A was not affected by Ser-154 phosphorylation. It is likely that the stabilization of cyclin A by the kinase-inactive CDKs was mainly due to a cell cycle effect. These data suggest an important difference between the regulation of cyclin A and cyclin E.

Nephrocystin: gene expression and sequence conservation between human, mouse, and Caenorhabditis elegans

Juvenile nephronophthisis, an autosomal recessive cystic kidney disease, is the primary genetic cause for chronic renal failure in children. The gene (NPHP1) for nephronophthisis type 1 has recently been identified. Its gene product, nephrocystin, is a novel protein of unknown function, which contains a src-homology 3 domain. To study tissue expression and analyze amino acid sequence conservation of nephrocystin, the full-length murine Nphp1 cDNA sequence was obtained and Northern and in situ hybridization analyses were performed for extensive expression studies. The results demonstrate widespread but relatively weak NPHP1 expression in the human adult. In the adult mouse there is strong expression in testis. This expression occurs specifically in cell stages of the first meiotic division and thereafter. In situ hybridization to whole mouse embryos demonstrated widespread and uniform expression at all developmental stages. Amino acid sequence conservation studies in human, mouse, and Caenorhabditis elegans show that in nephrocystin the src-homology 3 domain is embedded in a novel context of other putative domains of protein-protein interaction, such as coiled-coil and E-rich domains. It is concluded that for multiple putative protein-protein interaction domains of nephrocystin, sequence conservation dates back at least to Caenorhabditis elegans. The previously described discrepancy between widespread tissue expression and the restriction of symptoms to the kidney has now been confirmed by an in-depth expression study.

The murine SCP3 gene is required for synaptonemal complex assembly, chromosome synapsis, and male fertility

During meiosis, the homologous chromosomes pair and recombine. An evolutionarily conserved protein structure, the synaptonemal complex (SC), is located along the paired meiotic chromosomes. We have studied the function of a structural component in the axial/lateral element of the SC, the synaptonemal complex protein 3 (SCP3). A null mutation in the SCP3 gene was generated, and we noted that homozygous mutant males were sterile due to massive apoptotic cell death during meiotic prophase. The SCP3-deficient male mice failed to form axial/lateral elements and SCs, and the chromosomes in the mutant spermatocytes did not synapse. While the absence of SCP3 affected the nuclear distribution of DNA repair and recombination proteins (Rad51 and RPA), as well as synaptonemal complex protein 1 (SCP1), a residual chromatin organization remained in the mutant meiotic cells.

c-myb Transactivates the Human Cyclin A1 Promoter and Induces Cyclin A1 Gene Expression

Cyclin A1 differs from other cyclins in its highly restricted expression pattern. Besides its expression during spermatogenesis, cyclin A1 is also expressed in hematopoietic progenitor cells and in acute myeloid leukemia. We investigated mechanisms that might contribute to cyclin A1 expression in hematopoietic cells. Comparison of cyclin A1 and cyclin A promoter activity in adherent and myeloid leukemia cell lines showed that the cyclin A1 promoter is preferentially active in myeloid cell lines. This preferential activity was present in a small, 335-bp cyclin A1 promoter fragment that contained several potential c-myb binding sites. Coexpression of a c-myb expression vector with the cyclin A1 promoter constructs significantly increased the reporter activity in adherent CV-1 as well as in myeloid U937 cells. Gel-shift assays demonstrated that c-myb could bind to the cyclin A1 promoter at a binding site located near the transcription start site. Site-directed mutagenesis of this site decreased promoter transactivation by 50% in both KCL22 cells that express high levels of c-myb and in CV-1 cells that were transfected with c-myb. In addition, transfection of primary human embryonic fibroblasts with a c-myb expression vector led to induction of the endogenous cyclin A1 gene. Taken together, c-myb can directly transactivate the promoter of cyclin A1, and c-myb might be involved in the high-level expression of cyclin A1 observed in acute myeloid leukemia. These findings suggest that c-myb induces hematopoiesis-specific mechanisms of cell cycle regulation.

c-myb Transactivates the Human Cyclin A1 Promoter and Induces Cyclin A1 Gene Expression

Cyclin A1 differs from other cyclins in its highly restricted expression pattern. Besides its expression during spermatogenesis, cyclin A1 is also expressed in hematopoietic progenitor cells and in acute myeloid leukemia. We investigated mechanisms that might contribute to cyclin A1 expression in hematopoietic cells. Comparison of cyclin A1 and cyclin A promoter activity in adherent and myeloid leukemia cell lines showed that the cyclin A1 promoter is preferentially active in myeloid cell lines. This preferential activity was present in a small, 335-bp cyclin A1 promoter fragment that contained several potential c-myb binding sites. Coexpression of a c-myb expression vector with the cyclin A1 promoter constructs significantly increased the reporter activity in adherent CV-1 as well as in myeloid U937 cells. Gel-shift assays demonstrated that c-myb could bind to the cyclin A1 promoter at a binding site located near the transcription start site. Site-directed mutagenesis of this site decreased promoter transactivation by 50% in both KCL22 cells that express high levels of c-myb and in CV-1 cells that were transfected with c-myb. In addition, transfection of primary human embryonic fibroblasts with a c-myb expression vector led to induction of the endogenous cyclin A1 gene. Taken together, c-myb can directly transactivate the promoter of cyclin A1, and c-myb might be involved in the high-level expression of cyclin A1 observed in acute myeloid leukemia. These findings suggest that c-myb induces hematopoiesis-specific mechanisms of cell cycle regulation.

Cloning and expression analysis of a Petunia hybrida flower specific mitotic-like cyclin

A cyclin cDNA clone (Pethy;CycB1;1) was isolated from a Petunia hybrida ovary specific cDNA library. Sequence comparison revealed that Pethy;CYCB1;1 protein is highly homologous to mitotic B1 cyclins. Northern analysis and in situ hybridisation experiments showed that its expression is developmentally regulated and restricted to flower organs. We have attempted to define some of the cell division patterns which contribute to shaping each floral organ by analysing Pethy;CycB1;1 expression on Petunia flower sections. While in sepals, epidermis and parenchyma cell division patterns were comparable, there were two distinct cell division patterns in petals. In the epidermis, Pethy;CYCB1;1 expression was found both at the petal tip and along epidermis, whereas in the parenchyma only at the petal tips. In reproductive organs cell divisions were detected only in sporophytic tissues. No signals were detected inside meiotic cells.

Activation of the mitogen-activated protein kinase ERK1 during meiotic progression of mouse pachytene spermatocytes

Okadaic acid (OA) causes meiotic progression and chromosome condensation in cultured pachytene spermatocytes and an increase in maturation promoting factor (cyclin B1/cdc2 kinase) activity, as evaluated by H1 phosphorylative activity in anti-cyclin B1 immunoprecipitates. OA also induces a strong increase of phosphorylative activity toward the mitogen-activated protein kinase substrate myelin basic protein (MBP). Immunoprecipitation experiments with anti-extracellular signal-regulated kinase 1 (ERK1) or anti-ERK2 antibodies followed by MBP kinase assays, and direct in-gel kinase assays for MBP, show that p44/ERK1 but not p42/ERK2 is stimulated in OA-treated spermatocytes. OA treatment stimulates phosphorylation of ERK1, but not of ERK2, on a tyrosine residue involved in activation of the enzyme. ERK1 immunoprecipitated from extracts of OA-stimulated spermatocytes induces a stimulation of H1 kinase activity in extracts from control pachytene spermatocytes, whereas immunoprecipitated ERK2 is uneffective. We also show that natural G(2)/M transition in spermatocytes is associated to intracellular redistribution of ERKs, and their association with microtubules of the metaphase spindle. Preincubation of cultured pachytene spermatocytes with PD98059 (a selective inhibitor of ERK-activating kinases MEK1/2) completely blocks the ability of OA to induce chromosome condensation and progression to meiotic metaphases. These results suggest that ERK1 is specifically activated during G(2)/M transition in mouse spermatocytes, that it contributes to the mechanisms of maturation promoting factor activation, and that it is essential for chromosome condensation associated with progression to meiotic metaphases.

Completion of meiosis is not always required for acrosome formation in HSP70-2 null mice

Hsp70-2 is a unique member of the mouse 70-kDa heat shock protein family that is synthesized during meiosis in spermatogenic cells. Germ cells in male mice homozygous for a targeted mutation in the Hsp70-2 gene (Hsp70-2(-/-)) arrest in development and undergo apoptosis at the end of the pachytene spermatocyte stage of meiotic prophase. However, cells with a putative acrosome were present occasionally in histological sections of the testes of juvenile and adult Hsp70-2(-/-) mice. This study verified that acrosomes were present and investigated the relationship between acrosome formation and the process of meiosis. Histochemistry with the periodic acid-Schiff procedure and immunostaining with monoclonal antibody MN7 verified that acrosomes were present in Hsp70-2(-/-) mice, and electron microscopy showed that some of these cells had condensing nuclei characteristic of step 8-9 spermatids. The frequency of acrosome-containing cells in Hsp70-2(-/-) mice was less than 0.01% of that in wild-type mice. Propidium iodide staining and cytophotometry indicated that the average DNA content of nuclei in MN7-positive cells in Hsp70-2(-/-) mice was usually about twice, or occasionally the same as, that of nuclei in round spermatids of wild-type mice. Meiotic metaphase I and II chromosome spreads were observed in spermatogenic cells from Hsp70-2(-/-) mice but at a much lower frequency than in wild-type mice. These results indicate that not all pachytene spermatocytes in Hsp70-2(-/-) mice arrest in meiosis, but they may divide once or sometimes twice and begin acrosome formation and nuclear condensation. This demonstrates that some aspects of spermatid development can occur without the completion of meiosis in mice, as has been reported recently for Drosophila.

It's a knockout!

'It's a Knockout!' provides an update of some of the latest mouse knockouts in TBASE (http://www.jax.org/tbase/ and Ref. 1). The column provides a concise phenotypic profile of novel mutants and renders their complete characterization directly accessible to Web users, via unique and unchanging accession numbers (TBASE identities). Where possible, interesting knockouts will be grouped according to gene families, application or phenotypic similarities.