Autoregulation of Musashi1 mRNA translation during Xenopus oocyte maturation

The mRNA translational control protein, Musashi, plays a critical role in cell fate determination through sequence‐specific interactions with select target mRNAs. In proliferating stem cells, Musashi exerts repression of target mRNAs to promote cell cycle progression. During stem cell differentiation, Musashi target mRNAs are de‐repressed and translated. Recently, we have reported an obligatory requirement for Musashi to direct translational activation of target mRNAs during Xenopus oocyte meiotic cell cycle progression. Despite the importance of Musashi in cell cycle regulation, only a few target mRNAs have been fully characterized. In this study, we report the identification and characterization of a new Musashi target mRNA in Xenopus oocytes. We demonstrate that progesterone‐stimulated translational activation of the Xenopus Musashi1 mRNA is regulated through a functional Musashi binding element (MBE) in the Musashi1 mRNA 3′ untranslated region (3′ UTR). Mutational disruption of the MBE prevented translational activation of Musashi1 mRNA and its interaction with Musashi protein. Further, elimination of Musashi function through microinjection of inhibitory antisense oligonucleotides prevented progesterone‐induced polyadenylation and translation of the endogenous Musashi1 mRNA. Thus, Xenopus Musashi proteins regulate translation of the Musashi1 mRNA during oocyte maturation. Our results indicate that the hierarchy of sequential and dependent mRNA translational control programs involved in directing progression through meiosis are reinforced by an intricate series of nested, positive feedback loops, including Musashi mRNA translational autoregulation. These autoregulatory positive feedback loops serve to amplify a weak initiating signal into a robust commitment for the oocyte to progress through the cell cycle and become competent for fertilization.Mol. Reprod. Dev. 79: 553‐563, 2012. © 2012 Wiley Periodicals, Inc.     

Evidence that female preferences have shaped male signal evolution in a clade of specialized plant-feeding insects

Mate choice is considered an important influence in the evolution of mating signals and other sexual traits, and--since divergence in sexual traits causes reproductive isolation--it can be an agent of population divergence. The importance of mate choice in signal evolution can be evaluated by comparing male signal traits with female preference functions, taking into account the shape and strength of preferences. Specifically, when preferences are closed (favouring intermediate values), there should be a correlation between the preferred values and the trait means, and stronger preferences should be associated with greater preference-signal correspondence and lower signal variability. When preferences are open (favouring extreme values), signal traits are not only expected to be more variable, but should also be shifted towards the preferred values. We tested the role of female preferences in signal evolution in the Enchenopa binotata species complex of treehoppers, a clade of plant-feeding insects hypothesized to have speciated in sympatry. We found the expected relationship between signals and preferences, implicating mate choice as an agent of signal evolution. Because differences in sexual communication systems lead to reproductive isolation, the factors that promote divergence in female preferences--and, consequently, in male signals--may have an important role in the process of speciation.     

Glycans as receptors for influenza pathogenesis

Influenza A viruses, members of the Orthomyxoviridae family, are responsible for annual seasonal influenza epidemics and occasional global pandemics. The binding of viral coat glycoprotein hemagglutinin (HA) to sialylated glycan receptors on host epithelial cells is the critical initial step in the infection and transmission of these viruses. Scientists believe that a switch in the binding specificity of HA from Neu5Acα2-3Gal linked (α2-3) to Neu5Acα2-6Gal linked (α2-6) glycans is essential for the crossover of the viruses from avian to human hosts. However, studies have shown that the classification of glycan binding preference of HA based on sialic acid linkage alone is insufficient to establish a correlation between receptor specificity of HA and the efficient transmission of influenza A viruses. A recent study reported extensive diversity in the structure and composition of α2-6 glycans (which goes beyond the sialic acid linkage) in human upper respiratory epithelia and identified different glycan structural topologies. Biochemical examination of the multivalent HA binding to these diverse sialylated glycan structures also demonstrated that high affinity binding of HA to α2-6 glycans with a characteristic umbrella-like structural topology is critical for efficient human adaptation and human-human transmission of influenza A viruses. This review summarizes studies which suggest a new paradigm for understanding the role of the structure of sialylated glycan receptors in influenza virus pathogenesis.     

Differential effects of Heparitinase I and Heparitinase III on endothelial tube formation in vitro

Heparan sulfate proteoglycans (HSPGs) play vital roles in many steps of angiogenesis under physiological and pathological conditions. HSPGs on endothelial cell surfaces act as co-receptors for a variety of pro-angiogenic growth factors such as FGF and VEGF and anti-angiogenic factors such as endostatin. However, the fine structural requirements of these binding interactions are dependent on the sulfation patterns of HSPGs. Previous studies have shown that Heparitinases, heparin lyases isolated from Flavobacterium heparinum, can cleave heparan sulfate chains. These enzymes have been shown to reduce tumor—derived neovascularization in vivo in mice. However, the results from these experiments could not conclusively pinpoint the origin of the HS fragments. Thus, in this study we utilized an in vitro assay to assess the differential effects of Heparitinase I (Hep I) and Heparitinase III (Hep III) on endothelial tube formation. Hep III was found to be a more potent inhibitor of tube formation than Hep I. In conclusion, differential cleavage of endothelial cell surface bound HS can affect the extent of inhibition of tube formation.     

Cdc20 hypomorphic mice fail to counteract de novo synthesis of cyclin B1 in mitosis

Cdc20 is an activator of the anaphase-promoting complex/cyclosome that initiates anaphase onset by ordering the destruction of cyclin B1 and securin in metaphase. To study the physiological significance of Cdc20 in higher eukaryotes, we generated hypomorphic mice that express small amounts of this essential cell cycle regulator. In this study, we show that these mice are healthy and not prone to cancer despite substantial aneuploidy. Cdc20 hypomorphism causes chromatin bridging and chromosome misalignment, revealing a requirement for Cdc20 in efficient sister chromosome separation and chromosome-microtubule attachment. We find that cyclin B1 is newly synthesized during mitosis via cytoplasmic polyadenylation element-binding protein-dependent translation, causing its rapid accumulation between prometaphase and metaphase of Cdc20 hypomorphic cells. Anaphase onset is significantly delayed in Cdc20 hypomorphic cells but not when translation is inhibited during mitosis. These data reveal that Cdc20 is particularly rate limiting for cyclin B1 destruction because of regulated de novo synthesis of this cyclin after prometaphase onset.     

Overexpression of the E2 ubiquitin–conjugating enzyme UbcH10 causes chromosome missegregation and tumor formation

The anaphase-promoting complex/cyclosome (APC/C) E3 ubiquitin ligase functions with the E2 ubiquitin–conjugating enzyme UbcH10 in the orderly progression through mitosis by marking key mitotic regulators for destruction by the 26-S proteasome. UbcH10 is overexpressed in many human cancer types and is associated with tumor progression. However, whether UbcH10 overexpression causes tumor formation is unknown. To address this central question and to define the molecular and cellular consequences of UbcH10 overexpression, we generated a series of transgenic mice in which UbcH10 was overexpressed in graded fashion. In this study, we show that UbcH10 overexpression leads to precocious degradation of cyclin B by the APC/C, supernumerary centrioles, lagging chromosomes, and aneuploidy. Importantly, we find that UbcH10 transgenic mice are prone to carcinogen-induced lung tumors and a broad spectrum of spontaneous tumors. Our results identify UbcH10 as a prominent protooncogene that causes whole chromosome instability and tumor formation over a wide gradient of overexpression levels.     

Cdc20 Is Critical for Meiosis I and Fertility of Female Mice

Chromosome missegregation in germ cells is an important cause of unexplained infertility, miscarriages, and congenital birth defects in humans. However, the molecular defects that lead to production of aneuploid gametes are largely unknown. Cdc20, the activating subunit of the anaphase-promoting complex/cyclosome (APC/C), initiates sister-chromatid separation by ordering the destruction of two key anaphase inhibitors, cyclin B1 and securin, at the transition from metaphase to anaphase. The physiological significance and full repertoire of functions of mammalian Cdc20 are unclear at present, mainly because of the essential nature of this protein in cell cycle progression. To bypass this problem we generated hypomorphic mice that express low amounts of Cdc20. These mice are healthy and have a normal lifespan, but females produce either no or very few offspring, despite normal folliculogenesis and fertilization rates. When mated with wild-type males, hypomorphic females yield nearly normal numbers of fertilized eggs, but as these embryos develop, they become malformed and rarely reach the blastocyst stage. In exploring the underlying mechanism, we uncover that the vast majority of these embryos have abnormal chromosome numbers, primarily due to chromosome lagging and chromosome misalignment during meiosis I in the oocyte. Furthermore, cyclin B1, cyclin A2, and securin are inefficiently degraded in metaphase I; and anaphase I onset is markedly delayed. These results demonstrate that the physiologically effective threshold level of Cdc20 is high for female meiosis I and identify Cdc20 hypomorphism as a mechanism for chromosome missegregation and formation of aneuploid gametes.     

Determinants of glycan receptor specificity of H2N2 influenza A virus hemagglutinin

The H2N2 subtype of influenza A virus was responsible for the Asian pandemic of 1957-58. However, unlike other subtypes that have caused pandemics such as H1N1 and H3N2, which continue to circulate among humans, H2N2 stopped circulating in the human population in 1968. Strains of H2 subtype still continue to circulate in birds and occasionally pigs and could be reintroduced into the human population through antigenic drift or shift. Such an event is a potential global health concern because of the waning population immunity to H2 hemagglutinin (HA). The first step in such a cross-species transmission and human adaptation of influenza A virus is the ability for its surface glycoprotein HA to bind to glycan receptors expressed in the human upper respiratory epithelia. Recent structural and biochemical studies have focused on understanding the glycan receptor binding specificity of the 1957-58 pandemic H2N2 HA. However, there has been considerable HA sequence divergence in the recent avian-adapted H2 strains from the pandemic H2N2 strain. Using a combination of structural modeling, quantitative glycan binding and human respiratory tissue binding methods, we systematically identify mutations in the HA from a recent avian-adapted H2N2 strain (A/Chicken/PA/2004) that make its quantitative glycan receptor binding affinity (defined using an apparent binding constant) comparable to that of a prototypic pandemic H2N2 (A/Albany/6/58) HA.     

Integration of related sequences with protein three-dimensional structural families in an updated version of PALI database

The database of Phylogeny and ALIgnment of homologous protein structures (PALI) contains three-dimensional (3-D) structure-dependent sequence alignments as well as structure-based phylogenetic trees of protein domains in various families. The latest updated version (Release 2.1) comprises of 844 families of homologous proteins involving 3863 protein domain structures with each of these families having at least two members. Each member in a family has been structurally aligned with every other member in the same family using two proteins at a time. In addition, an alignment of multiple structures has also been performed using all the members in a family. Every family with at least three members is associated with two dendrograms, one based on a structural dissimilarity metric and the other based on similarity of topologically equivalenced residues for every pairwise alignment. Apart from these multi-member families, there are 817 single member families in the updated version of PALI. A new feature in the current release of PALI is the integration, with 3-D structural families, of sequences of homologues from the sequence databases. Alignments between homologous proteins of known 3-D structure and those without an experimentally derived structure are also provided for every family in the enhanced version of PALI. The database with several web interfaced utilities can be accessed at: http://pauling.mbu.iisc.ernet.in/~pali.