Loss of dmrt1 restores zebrafish female fates in the absence of cyp19a1a but not rbpms2a/b

Sex determination and differentiation is a complex process regulated by multiple factors, including factors from the germline or surrounding somatic tissue. In zebrafish, sex-determination involves establishment of a bipotential ovary that undergoes sex-specific differentiation and maintenance to form the functional adult gonad. However, the relationships among these factors are not fully understood. Here, we identify potential Rbpms2 targets and apply genetic epistasis experiments to decipher the genetic hierarchy of regulators of sex-specific differentiation. We provide genetic evidence that the crucial female factor rbpms2 is epistatic to the male factor dmrt1 in terms of adult sex. Moreover, the role of Rbpms2 in promoting female fates extends beyond repression of Dmrt1, as Rbpms2 is essential for female differentiation even in the absence of Dmrt1. In contrast, female fates can be restored in mutants lacking both cyp19a1a and dmrt1, and prolonged in bmp15 mutants in the absence of dmrt1. Taken together, this work indicates that cyp19a1a-mediated suppression of dmrt1 establishes a bipotential ovary and initiates female fate acquisition. Then, after female fate specification, Cyp19a1a regulates subsequent oocyte maturation and sustains female fates independently of Dmrt1 repression.

RINGO/Speedy proteins, a family of non-canonical activators of CDK1 and CDK2

Cyclin-dependent kinases (CDKs) require the binding to a regulatory subunit to acquire enzymatic activity, and cyclins are the canonical CDK activators. However, there are specific situations in which CDKs can be activated by non-cyclin proteins that are less characterized. This review focuses on the family of RINGO/Speedy proteins, which have no sequence amino acid homology to cyclins but can bind to and activate CDK1 and CDK2. Interestingly, RINGO/Speedy proteins can activate CDKs under conditions in which CDK-cyclin complexes would not be active, and there is evidence that RINGO/Speedy-activated CDKs can phosphorylate different sites than the cyclin-activated CDKs. RINGO/Speedy proteins were originally described in Xenopus oocytes, but their roles in mammalian cells have also been addressed. We will summarize the properties of RINGO/Speedy proteins and how they trigger CDK activation, and discuss recent studies that characterized their physiological functions. In particular, studies using genetically modified mice have shown that RingoA, also known as Spy1, plays a key role in meiosis regulation. Emerging evidence also suggests a potential role for RingoA/Spy1 in cancer.

Triggering mitosis

Entry into mitosis is triggered by the activation of cyclin-dependent kinase 1 (Cdk1). This simple reaction rapidly and irreversibly sets the cell up for division. Even though the core step in triggering mitosis is so simple, the regulation of this cellular switch is highly complex, involving a large number of interconnected signalling cascades. We do have a detailed knowledge of most of the components of this network, but only a poor understanding of how they work together to create a precise and robust system that ensures that mitosis is triggered at the right time and in an orderly fashion. In this review, we will give an overview of the literature that describes the Cdk1 activation network and then address questions relating to the systems biology of this switch. How is the timing of the trigger controlled? How is mitosis insulated from interphase? What determines the sequence of events, following the initial trigger of Cdk1 activation? Which elements ensure robustness in the timing and execution of the switch? How has this system been adapted to the high levels of replication stress in cancer cells?

Expression profile and potential functional differentiation of the Speedy/RINGO family in mice

As novel cyclin-dependent kinase (CDK) activators, Speedy/RINGO (hereafter named Speedy) proteins can directly regulate the cell cycle of vertebrates by binding to and activating various CDKs. Previous studies have shown that Speedy genes are highly associated with different types of cancer and other diseases. However, Speedy genes have not been systematically identified in mice, and their function and expression profiles remain elusive, which greatly hinders the functional and mechanistic study of Speedy genes in vivo. Here, we comprehensively identified Speedy genes in the mouse genome. Phylogenetic analysis showed that the Speedy gene family should be divided into three subfamilies, rather than the previously reported two subfamilies. Mice have two of the three subfamilies of Speedy genes, namely, subfamilies A and E. Speedy subfamily C genes have been lost from the mouse genome. By combining experimental and bioinformatics approaches, we found that the genes from subfamilies A and E have different expression profiles, indicating their functional divergence, which was also consistent with the phylogenetic results. The genes belonging to subfamily E showed only slightly different expression profiles, indicating their similar functions. Coexpression network analysis showed that the genes coexpressed with mouse Speedy genes were primarily enriched in reproduction-related mechanisms and there were significant functional differences between genes from subfamilies A and E, further demonstrating functional differentiation. In summary, we provide a comprehensive landscape (from evolution to expression and function) of the Speedy family in mice; we also demonstrate that Speedy genes mainly participate in reproduction-related mechanisms and that they have undergone functional differentiation in mice.

A CD4+ T cell antagonist epitope down-regulates activating signaling proteins, up-regulates inhibitory signaling proteins and abrogates HIV-specific T cell function

Background

CD4⁺ T cells are critically important in HIV infection, being both the primary cells infected by HIV and likely playing a direct or indirect role in helping control virus replication. Key areas of interest in HIV vaccine research are mechanisms of viral escape from the immune response. Interestingly, in HIV infection it has been shown that peptide sequence variation can reduce CD4⁺ T cell responses to the virus, and small changes to peptide sequences can transform agonist peptides into antagonist peptides.

Results

We describe, at a molecular level, the consequences of antagonism of HIV p24-specific CD4⁺ T cells. Antagonist peptide exposure in the presence of agonist peptide caused a global suppression of agonist-induced gene expression and signaling molecule phosphorylation. In addition to down-regulation of factors associated with T cell activation, a smaller subset of genes associated with negative regulation of cell activation was up-regulated, including KFL-2, SOCS-1, and SPDEY9P. Finally, antagonist peptide in the absence of agonist peptide also delivered a negative signal to T cells.

Conclusions

Small changes in p24-specific peptides can result in T cell antagonism and reductions of both T cell receptor signaling and activation. These changes are at least in part mediated by a dominant negative signal delivered by antagonist peptide, as evidenced by up-regulation of negative regulatory genes in the presence of agonist plus antagonist stimulation. Antagonism can have dramatic effects on CD4⁺ T cell function and presents a potential obstacle to HIV vaccine development.

CDK2-dependent phosphorylation of Suv39H1 is involved in control of heterochromatin replication during cell cycle progression

Although several studies have suggested that the functions of heterochromatin regulators may be regulated by post-translational modifications during cell cycle progression, regulation of the histone methyltransferase Suv39H1 is not fully understood. Here, we demonstrate a direct link between Suv39H1 phosphorylation and cell cycle progression. We show that CDK2 phosphorylates Suv39H1 at Ser391 and these phosphorylation levels oscillate during the cell cycle, peaking at S phase and maintained during S-G2-M phase. The CDK2-mediated phosphorylation of Suv39H1 at Ser391 results in preferential dissociation from chromatin. Furthermore, phosphorylation-mediated dissociation of Suv39H1 from chromatin causes an enhanced occupancy of JMJD2A histone demethylase on heterochromatin and alterations in inactive histone marks. Overexpression of phospho-mimic Suv39H1 induces early replication of heterochromatin, suggesting the importance of Suv39H1 phosphorylation in the replication of heterochromatin. Moreover, overexpression of phospho-defective Suv39H1 caused altered replication timing of heterochromatin and increases sensitivity to replication stress. Collectively, our data suggest that phosphorylation-mediated modulation of Suv39H1-chromatin association may be an initial step in heterochromatin replication.

Evolution of the Cdk-activator Speedy/RINGO in vertebrates

Successful completion of the cell cycle relies on the precise activation and inactivation of cyclin-dependent kinases (Cdks) whose activity is mainly regulated by binding to cyclins. Recently, a new family of Cdk regulators termed Speedy/RINGO has been discovered, which can bind and activate Cdks but shares no apparent amino acid sequence homology with cyclins. All Speedy proteins share a conserved domain of approximately 140 amino acids called "Speedy Box", which is essential for Cdk binding. Speedy/RINGO proteins display an important role in oocyte maturation in Xenopus. Interestingly, a common feature of all Speedy genes is their predominant expression in testis suggesting that meiotic functions may be the most important physiological feature of Speedy genes. Speedy homologs have been reported in mammals and can be traced back to the most primitive clade of chordates (Ciona intestinalis). Here, we investigated the evolution of the Speedy genes and have identified a number of new Speedy/RINGO proteins. Through extensive analysis of numerous species, we discovered diverse evolutionary histories: the number of Speedy genes varies considerably among species, with evidence of substantial gains and losses. Despite the interspecies variation, Speedy is conserved among most species examined. Our results provide a complete picture of the Speedy gene family and its evolution.

Cell cycle regulation of the mammalian CDK activator RINGO/Speedy A

Cell cycle progression is regulated by cyclin-dependent kinases (CDKs), whose activation requires the binding of regulatory subunits named cyclins. RINGO/Speedy A is a mammalian protein that has no amino acid sequence homology with cyclins but can activate CDKs. Here we show that RINGO/Speedy A is a highly unstable protein whose expression and phosphorylation are periodically regulated during the cell cycle. RINGO/Speedy A is degraded by the proteasome and the process involves the ubiquitin ligase SCF(Skp2). Overexpression of a stabilized RINGO/Speedy A form results in the accumulation of high levels of RINGO/Speedy A at late stages of mitosis, which interfere with cytokinesis and chromosome decondensation. Our data show that tight regulation of RINGO/Speedy A is important for the somatic cell cycle.