Critical differences between isoforms of securin reveal mechanisms of separase regulation

SPTBN1 Mediates the Cytoplasmic Constraint of PTTG1, Impairing Its Oncogenic Activity in Human Seminoma

Seminoma is the most common testicular cancer. Pituitary tumor-transforming gene 1 (PTTG1) is a securin showing oncogenic activity in several tumors. We previously demonstrated that nuclear PTTG1 promotes seminoma tumor invasion through its transcriptional activity on matrix metalloproteinase 2 (MMP-2) and E-cadherin (CDH1). We wondered if specific interactors could affect its subcellular distribution. To this aim, we investigated the PTTG1 interactome in seminoma cell lines showing different PTTG1 nuclear levels correlated with invasive properties. A proteomic approach upon PTTG1 immunoprecipitation uncovered new specific securin interactors. Western blot, confocal microscopy, cytoplasmic/nuclear fractionation, sphere-forming assay, and Atlas database interrogation were performed to validate the proteomic results and to investigate the interplay between PTTG1 and newly uncovered partners. We observed that spectrin beta-chain (SPTBN1) and PTTG1 were cofactors, with SPTBN1 anchoring the securin in the cytoplasm. SPTBN1 downregulation determined PTTG1 nuclear translocation, promoting its invasive capability. Moreover, a PTTG1 deletion mutant lacking SPTBN1 binding was strongly localized in the nucleus. The Atlas database revealed that seminomas that contained higher nuclear PTTG1 levels showed significantly lower SPTBN1 levels in comparison to non-seminomas. In human seminoma specimens, we found a strong PTTG1/SPTBN1 colocalization that decreases in areas with nuclear PTTG1 distribution. Overall, these results suggest that SPTBN1, along with PTTG1, is a potential prognostic factor useful in the clinical management of seminoma.

DNA damage checkpoint execution and the rules of its disengagement

Chromosomes are susceptible to damage during their duplication and segregation or when exposed to genotoxic stresses. Left uncorrected, these lesions can result in genomic instability, leading to cells’ diminished fitness, unbridled proliferation or death. To prevent such fates, checkpoint controls transiently halt cell cycle progression to allow time for the implementation of corrective measures. Prominent among these is the DNA damage checkpoint which operates at G2/M transition to ensure that cells with damaged chromosomes do not enter the mitotic phase. The execution and maintenance of cell cycle arrest are essential aspects of G2/M checkpoint and have been studied in detail. Equally critical is cells’ ability to switch-off the checkpoint controls after a successful completion of corrective actions and to recommence cell cycle progression. Interestingly, when corrective measures fail, cells can mount an unusual cellular response, termed adaptation, where they escape checkpoint arrest and resume cell cycle progression with damaged chromosomes at the cost of genome instability or even death. Here, we discuss the DNA damage checkpoint, the mitotic networks it inhibits to prevent segregation of damaged chromosomes and the strategies cells employ to quench the checkpoint controls to override the G2/M arrest.

Unravelling the diagnostic and therapeutic potentialities of a novel RNA aptamer isolated against human pituitary tumour transforming gene 1 (PTTG1) protein

Human Pituitary Tumour Transforming Gene 1 (PTTG1) is an oncoprotein involved in maintaining chromosome stability and acts as a biomarker for a panel of cancers. In this study, we endeavoured to generate an RNA aptamer against PTTG1. The RNA aptamer, SECURA-3 has an estimated equilibrium dissociation constant of 16.41 ± 6.4 nM. The aptamer was successfully harnessed in several diagnostic platforms including ELASA, aptamer-based dot blot and aptamer-based western blot. SECURA-3 was also unveiled as a potential probe that could replace its counterpart antibody in the histostaining-based detection of PTTG1 in HeLa and MCF-7 formalin-fixed paraffin-embedded cell blocks. In the aspect of therapeutics, SECURA-3 RNA aptamer demonstrates an antagonistic effect by antagonizing the interaction between PTTG1 and CXCR2, as revealed in the in vitro competitive nitrocellulose filter binding assay and dual-luciferase reporter assay in HeLa cells. As the first anti-PTTG1 aptamer, SECURA-3 RNA aptamer has immense diagnostic and therapeutic properties.

Complex Analysis of Retroposed Genes' Contribution to Human Genome, Proteome and Transcriptome

Gene duplication is a major driver of organismal evolution. One of the main mechanisms of gene duplications is retroposition, a process in which mRNA is first transcribed into DNA and then reintegrated into the genome. Most gene retrocopies are depleted of the regulatory regions. Nevertheless, examples of functional retrogenes are rapidly increasing. These functions come from the gain of new spatio-temporal expression patterns, imposed by the content of the genomic sequence surrounding inserted cDNA and/or by selectively advantageous mutations, which may lead to the switch from protein coding to regulatory RNA. As recent studies have shown, these genes may lead to new protein domain formation through fusion with other genes, new regulatory RNAs or other regulatory elements. We utilized existing data from high-throughput technologies to create a complex description of retrogenes functionality. Our analysis led to the identification of human retroposed genes that substantially contributed to transcriptome and proteome. These retrocopies demonstrated the potential to encode proteins or short peptides, act as cis- and trans- Natural Antisense Transcripts (NATs), regulate their progenitors’ expression by competing for the same microRNAs, and provide a sequence to lncRNA and novel exons to existing protein-coding genes. Our study also revealed that retrocopies, similarly to retrotransposons, may act as recombination hot spots. To our best knowledge this is the first complex analysis of these functions of retrocopies.

Biology and insights into the role of cohesin protease separase in human malignancies

Separase, an enzyme that resolves sister chromatid cohesion during the metaphase-to-anaphase transition, plays a pivotal role in chromosomal segregation and cell division. Separase protein, encoded by the extra spindle pole bodies like 1 (ESPL1) gene, is overexpressed in numerous human cancers including breast, bone, brain, and prostate. Separase is oncogenic, and its overexpression is sufficient to induce mammary tumours in mice. Either acute or chronic overexpression of separase in mouse mammary glands leads to aneuploidy and tumorigenesis, and inhibition of separase enzymatic activity decreases the growth of human breast tumour xenografts in mice. This review focuses on the biology of and insights into the molecular mechanisms of separase as an oncogene, and its significance and implications for human cancers.

Separase: Function Beyond Cohesion Cleavage and an Emerging Oncogene

Proper and timely segregation of genetic endowment is necessary for survival and perpetuation of every species. Mis-segregation of chromosomes and resulting aneuploidy leads to genetic instability, which can jeopardize the survival of an individual or population as a whole. Abnormality with segregation of genetic contents has been associated with several medical consequences including cancer, sterility, mental retardation, spontaneous abortion, miscarriages, and other birth related defects. Separase, by irreversible cleavage of cohesin complex subunit, paves the way for metaphase/anaphase transition during the cell cycle. Both over or reduced expression and altered level of separase have been associated with several medical consequences including cancer, as a result separase now emerges as an important oncogene and potential molecular target for medical intervenes. Recently, separase is also found to be essential in separation and duplication of centrioles. Here, I review the role of separase in mitosis, meiosis, non-canonical roles of separase, separase regulation, as a regulator of centriole disengagement, nonproteolytic roles, diverse substrates, structural insights, and association of separase with cancer. At the ends, I proposed a model which showed that separase is active throughout the cell cycle and there is a mere increase in separase activity during metaphase contrary to the common believes that separase is inactive throughout cell cycle except for metaphase. J. Cell. Biochem. 118: 1283-1299, 2017. © 2016 Wiley Periodicals, Inc.

SGO1C is a non-functional isoform of Shugoshin and can disrupt sister chromatid cohesion by interacting with PP2A-B56

Shugoshin (SGO1) plays a pivotal role in sister chromatid cohesion during mitosis by protecting the centromeric cohesin from mitotic kinases and WAPL. Mammalian cells contain at least 6 alternatively spliced isoforms of SGO1. The relationship between the canonical SGO1A with shorter isoforms including SGO1C remains obscure. Here we show that SGO1C was unable to replace the loss of SGO1A. Instead, expression of SGO1C alone induced aberrant mitosis similar to depletion of SGO1A, promoting premature sister chromatid separation, activation of the spindle-assembly checkpoint, and mitotic arrest. In disagreement with previously published data, we found that SGO1C localized to kinetochores. However, the ability to induce aberrant mitosis did not correlate with its kinetochore localization. SGO1C mutants that abolished binding to kinetochores still triggered premature sister chromatid separation. We provide evidence that SGO1C-mediated mitotic arrest involved the sequestering of PP2A-B56 pool. Accordingly, SGO1C mutants that abolished binding to PP2A localized to kinetochores but did not induce aberrant mitosis. These studies imply that the expression of SGO1C should be tightly regulated to prevent dominant-negative effects on SGO1A and genome instability.

Structural Insights into Separase Architecture and Substrate Recognition through Computational Modelling of Caspase-Like and Death Domains

Separases are large proteins that mediate sister chromatid disjunction in all eukaryotes. They belong to clan CD of cysteine peptidases and contain a well-conserved C-terminal catalytic protease domain similar to caspases and gingipains. However, unlike other well-characterized groups of clan CD peptidases, there are no high-resolution structures of separases and the details of their regulation and substrate recognition are poorly understood. Here we undertook an in-depth bioinformatical analysis of separases from different species with respect to their similarity in amino acid sequence and protein fold in comparison to caspases, MALT-1 proteins (mucosa-associated lymphoidtissue lymphoma translocation protein 1) and gingipain-R. A comparative model of the single C-terminal caspase-like domain in separase from C. elegans suggests similar binding modes of substrate peptides between these protein subfamilies, and enables differences in substrate specificity of separase proteins to be rationalised. We also modelled a newly identified putative death domain, located N-terminal to the caspase-like domain. The surface features of this domain identify potential sites of protein-protein interactions. Notably, we identified a novel conserved region with the consensus sequence WWxxRxxLD predicted to be exposed on the surface of the death domain, which we termed the WR motif. We envisage that findings from our study will guide structural and functional studies of this important protein family.

The separation of sister chromatids is a crucial step in cell division and is triggered by the activation of separase, a protease that cleaves the proteins that maintain the cohesion between sister chromatids. Knowledge of the molecular structure and activation mechanism of separase is limited by the difficulty of obtaining structural information on this large and flexible protein. Sequence conservation between separase homologues from diverse species is limited to the C-terminal region that contains the catalytically active protease domain. We conducted an in-depth bioinformatical analysis of separase and generated structural models of the two conserved domains that comprise the C-terminal region: a caspase-like domain and a putative death domain. This analysis provided insights into substrate recognition and identified potential sites of protein-protein interactions. Both the death domain and caspase-like domain are well-conserved in separases, which suggests an evolutionary pressure to keep these two domains together, perhaps to enable separase activity and/or provide stability. Insights into the molecular structures of separase gained in this study may provide a starting point for experimental structural studies on this protein and may aid therapeutic development against cancers where chromosomes are improperly segregated.

Pituitary tumor transforming gene PTTG2 induces psoriasis by regulating vimentin and E-cadherin expression

Psoriasis is a common and intractable skin disease affecting the physical and mental health of patients. This study focused on the roles of pituitary tumor transforming gene 2 (PTTG2) in psoriasis. Using real-time quantitative PCR and western blot, the expression patterns of PTTG2 were compared in psoriatic epidermis cells and normal cells, from both mRNA levels and protein levels. Knockdown of PTTG2 by siRNA was conducted in HaCaT cells to investigate the changes in cell viability and migration in vitro. Expression changes of vimentin and E-cadherin were also detected in the transfected cells. Results showed PTTG2 was significantly overexpressed in the psoriatic epidermis cells (P < 0.05). The cell viability and migration were inhibited by the knockdown of PTTG2. Besides, knockdown of PTTG2 resulted in down-regulation of vimentin and up-regulation of E-cadherin, with significant differences compared to the siRNA control group (P < 0.05). This study indicated the involvement of PTTG2 in mediating epidermis cell viability and migration and in pathogenesis of psoriasis. PTTG2 might be a potential therapeutic target for psoriasis through inducing epithelial-to-mesenchymal transition (EMT) via regulating the expression of vimentin and E-cadherin.