The double life of Irs

The liver plays a central role in lipid and glucose metabolism. Two studies in this issue (Kubota et al., 2008; Dong et al., 2008) on the insulin-signaling adaptors Irs1 and Irs2 prompt a critical reappraisal of the physiology of fasting and of the integrated control of hepatic insulin action.     

Differential Impact of Lipoxygenase 2 and Jasmonates on Natural and Stress-Induced Senescence in Arabidopsis

Jasmonic acid and related oxylipins are controversially discussed to be involved in regulating the initiation and progression of leaf senescence. To this end, we analyzed profiles of free and esterified oxylipins during natural senescence and upon induction of senescence-like phenotypes by dark treatment and flotation on sorbitol in Arabidopsis (Arabidopsis thaliana). Jasmonic acid and free 12-oxo-phytodienoic acid increased during all three processes, with the strongest increase of jasmonic acid after dark treatment. Arabidopside content only increased considerably in response to sorbitol treatment. Monogalactosyldiacylglycerols and digalactosyldiacylglycerols decreased during these treatments and aging. Lipoxygenase 2-RNA interference (RNAi) plants were generated, which constitutively produce jasmonic acid and 12-oxo-phytodienoic acid but do not exhibit accumulation during natural senescence or upon stress treatment. Chlorophyll loss during aging and upon dark incubation was not altered, suggesting that these oxylipins are not involved in these processes. In contrast, lipoxygenase 2-RNAi lines and the allene oxid synthase-deficient mutant dde2 were less sensitive to sorbitol than the wild type, indicating that oxylipins contribute to the response to sorbitol stress.Senescence is an important, highly regulated process at the end of development. Senescence is characterized by breakdown of organelles and molecules, export and transport of these nutrients to other organs/parts of the organism, and finally programmed cell death of the senescing organ.The process of senescence has been intensively studied in leaves, and morphological as well as molecular changes in senescing leaves have been described. Yellowing as a consequence of chlorophyll and chloroplast degradation is the most obvious process during natural leaf senescence. In addition, gene expression changes dramatically during senescence. Some senescence-associated genes (SAG, SEN) have been reported that are induced during this process, and several of the encoded proteins function in macromolecule degradation, detoxification and defense metabolism, or signal transduction (Gepstein et al., 2003). Based on the degradation of chloroplasts and macromolecules, leaf metabolism changes from carbon assimilation to catabolism (Lim et al., 2007).The initiation and progression of senescence is regulated by endogenous as well as exogenous factors. Among the endogenous factors, the developmental status of the organ and of the whole plant (e.g. age and progress in flowering and seed production) has a great impact on the process of senescence. Different stress factors such as pathogen attack, drought, osmotic stress, heat, cold, ozone, UV light, and shading can induce or accelerate senescence (Quirino et al., 2000). Phytohormones are very important regulators that integrate information about the developmental status and the environmental factors. Cytokinins are antagonistic signals and delay senescence. Endogenous levels of cytokinins decrease during senescence, and exogenous application and transgenic approaches, enhancing endogenous levels of these compounds, lead to delayed senescence (Gan and Amasino, 1995). In contrast, the gaseous phytohormone ethylene is known to induce and accelerate senescence (John et al., 1995). There are also several indications that abscisic acid modulates senescence (van der Graaff et al., 2006). The roles of other phytohormones/signaling compounds such as auxin, salicylic acid, and jasmonates are less clear (Lim et al., 2007).Jasmonates are oxylipin signaling molecules derived from linolenic acid. The term jasmonates comprises 12-oxo-phytodienoic acid (OPDA), jasmonic acid (JA), and derivatives such as the methyl ester and amino acid conjugates of JA. One of the first biological activities described for these compounds was the promotion of senescence in oat (Avena sativa) leaves by methyl jasmonate (MeJa) isolated from Artemisia absinthium (Ueda and Kato, 1980). Later on, the induction of senescence-like phenotypes by exogenous application of MeJa was also found in other plant species (Ueda and Kato, 1980; Weidhase et al., 1987a; He et al., 2002). On the molecular level, this senescence-promoting effect of MeJa is accompanied by chlorophyll loss and decreases in Rubisco and photosynthesis (Weidhase et al., 1987a, 1987b). In addition, expression of some senescence-up-regulated genes is also responsive to JA; examples are SEN1, SEN4, SEN5, SAG12, SAG14, and SAG15 (Park et al., 1998; Schenk et al., 2000; He et al., 2002). Due to the results described above, jasmonates have been described for decades as compounds with senescence-promoting activities, while the function of these compounds in natural senescence in planta was critically discussed (Parthier, 1990; Sembdner and Parthier, 1993; Creelman and Mullet, 1997; Wasternack, 2007; Balbi and Devoto, 2008; Reinbothe et al., 2009). Additional indications for a role of jasmonates in regulating senescence are the transient up-regulation of expression of some enzymes involved in JA biosynthesis, such as allene oxide synthase (AOS) and OPDA reductase 3 (OPR3), and the increase in JA levels during natural senescence (He et al., 2002; van der Graaff et al., 2006). Furthermore, alterations in natural and induced senescence have been reported for some mutants with defects in the JA pathway. The mutant coi1, which is impaired in JA signaling, exhibited delayed chlorophyll loss upon dark incubation of detached leaves (Castillo and Leon, 2008). Plants with reduced expression of the 3-ketoacyl-CoA-thiolase KAT2, which is involved in β-oxidation and JA production, showed delayed yellowing during natural senescence and upon dark incubation of detached leaves (Castillo and Leon, 2008).However, there are also several reports that cast doubt on an important function of JA in senescence. For most mutants in JA biosynthesis or signaling, no differences in natural senescence are apparent (He et al., 2002; Schommer et al., 2008). In addition, mutants defective in the expression of AOS or OPR3 do not show altered senescence-like phenotypes upon dark treatment (Schommer et al., 2008; Kunz et al., 2009). It has to be taken into consideration that the knockout in these mutants has pleiotrophic effects during whole plant development. For example, the leaves of plants with reduced expression of the lipase DGL or of OPR3 are larger (Hyun et al., 2008). In addition, several knockout mutants defective in JA biosynthesis or signaling do not produce fertile flowers (Feys et al., 1994; McConn and Browse, 1996; Sanders et al., 2000; Stintzi and Browse, 2000; Ishiguro et al., 2001; von Malek et al., 2002). These changes in development might affect other developmental processes such as senescence.To investigate the function of jasmonates in senescence in more detail, we compared the oxylipin profile of wild-type leaves during natural senescence and upon stress induction of senescence-like phenotypes. The analysis of lipoxygenase 2 (LOX2)-RNA interference (RNAi) plants, which produce low basal levels of oxylipins but are impaired in the accumulation of OPDA and JA during senescence or in response to stress, indicates that 13-LOX products are not necessary for natural senescence or dark-induced chlorophyll loss but are involved in the response to sorbitol.     

The Yin and Yang of treating BRCA-deficient tumors

The myriad changes that occur during the malignant progression of cancer cells present challenges to both clinicians and basic scientists. Two new studies in Nature underscore the central role of genome instability in tumor biology (Edwards et al., 2008; Sakai et al., 2008). These reports describe secondary changes in the BRCA2 locus that restore the wild-type reading frame and contribute to the development of resistance to chemotherapeutic agents.     

Epigenetic Variations in Plant Hybrids and Their Potential Roles in Heterosis

Heterosis,one of the most important biological phenomena,refers to the phenotypic superiority of a hybrid over its genetically diverse parents with respect to many traits such as biomass,growth rate and yield.Despite its successful application in breeding and agronomic production of many crop and animal varieties,the molecular basis of heterosis remains elusive.The classic genetic explanations for heterosis centered on three hypotheses:dominance (Davenport,1908;Bruce,1910;Keeble and Pellew,1910;Jones,1917),overdominance (East,1908;Shull,1908) and epistasis (Powers,1944;Yu et al.,1997).However,these hypotheses are largely conceptual and not connected to molecular principles,and are therefore insufficient to explain the molecular basis of heterosis (Birchler et al.,2003).Recently,many studies have explored the molecular mechanism of heterosis in plants at a genome-wide level.These studies suggest that global differential gene expression between hybrids and parental lines potentially contributes to heterosis in plants (e.g.,Swanson-Wagner et al.,2006;Zhang et al.,2008;Wei et al.,2009;Song et al.,2010).Research suggests that genetic components,including cis-acting elements and trans-acting factors,are critical regulators of differential gene expression in hybrids (Hochholdinger and Hoecker,2007;Springer and Stupar,2007;Zhang et al.,2008).However,other research indicates that epigenetic components,the regulators of chromatin states and genome activity,also have the potential to impact heterosis (e.g.,Ha et al.,2009;He et al.,2010;Groszmann et al.,2011;Barber et al.,2012;Chodavarapu et al.,2012;Greaves et al.,2012a;Shen et al.,2012).     

Senescence: not just for tumor suppression

Cellular senescence provides an intrinsic barrier to tumor development by preventing the proliferation of cells that are at risk for malignant transformation. In this issue, Krizhanovsky et al. (2008) report that senescence is an important player not only in tumor suppression but also in the response of liver tissue to injury.     

A tale of too many centrosomes

Having the correct number of centrosomes is crucial for proper chromosome segregation during cell division and for the prevention of aneuploidy, a hallmark of many cancer cells. Several recent studies (Basto et al., 2008; Kwon et al., 2008; Yang et al., 2008) reveal the importance of mechanisms that protect against the consequences of harboring too many centrosomes.     

A SNO storm in skeletal muscle

Dysregulated S-nitrosylation of proteins characterizes a broad array of human disorders, but its role in disease etiology is not well understood. Two new studies (Durham et al., 2008; Bellinger et al., 2008) now show that hyper-S-nitrosylation of the ryanodine receptor calcium release channel (RyR1) in skeletal muscle disrupts calcium ion flux. This disruption underlies the impaired contractility and cellular damage of skeletal muscle during strenuous exercise and in a spectrum of congenital muscle disorders including malignant hyperthermia.     

A novel role for Greatwall kinase in recovery from DNA damage

Activation of the DNA damage response (DDR) is critical for genomic integrity and tumor suppression. The occurrence of DNA damage quickly evokes the DDR through ATM/ATR-dependent signal transduction, which promotes DNA repair and activates the checkpoint to halt cell cycle progression (Halazonetis et al., 2008; Motoyama and Naka, 2004; Zhou and Elledge, 2000). The "turn off" process of the DDR upon satisfaction of DNA repair, also known as "checkpoint recovery", involves deactivation of DDR elements, but the mechanism is poorly understood. Greatwall kinase (Gwl) has been identified as a key element in the G2/M transition (Archambault et al., 2007; Jackson, 2006; Zhao et al., 2008; Yu et al., 2004; Yu et al., 2006; Zhao et al., 2006) and helps maintain M phase through inhibition of PP2A/B55δ (Burgess et al., 2010; Castilho et al., 2009; Goldberg, 2010; Lorca et al., 2010; Vigneron et al., 2009), the principal phosphatase for Cdk-phosphorylated substrates. Here we show that Gwl also promotes recovery from DNA damage and is itself directly inhibited by the DNA damage response (DDR). In Xenopus egg extracts, immunodepletion of Gwl increased the DDR to damaged DNA, whereas addition of wild type, but not kinase dead Gwl, inhibited the DDR. The removal of damaged DNA from egg extracts leads to recovery from checkpoint arrest and entry into mitosis, a process impaired by Gwl depletion and enhanced by Gwl over-expression. Moreover, activation of Cdk1 after the removal of damaged DNA is regulated by Gwl. Collectively, these results defines Gwl as a new regulator of the DDR, which plays an important role in recovery from DNA     

FOXO3-engineered human mesenchymal progenitor cells efficiently promote cardiac repair after myocardial infarction

Dear Editor,Myocardial infarction(MI)is the irreversible cardiomyocyte death resulting from prolonged oxygen deprivation due to obstructed blood supply(ischemia),leading to contractile dysfunction and cardiac remodeling.In recent decades,stem cell transplantation has been extensively investigated for the repair of injured heart in animal studies and clinical trials(Kanelidis et al.,2017;Gyongyosi et al.,2018).     

JAK2 Translocations in hematological malignancies: Review of the literature

JAK2 is a cytoplasmic tyrosine kinase whose gene is located on chromosome 9p24. It is involved in the regulation of different cytokines and growth factors and plays an important role in the diagnosis and treatment of myeloproliferative neoplasms (Smith et al., 2008). Translocations involving the JAK2 locus are uncommon with just a few cases described in the literature, and they usually lead to a fusion protein with JAK2 (Patnaik et al., 2010). Chromosome 9p24 abnormalities have been described in myeloid and lymphoid neoplasms including chronic myelogenous leukemia (CML), acute megakaryoblastic leukemia, CD10+ B-cell acute lymphoblastic leukemia, T-cell ALL and chronic myeloproliferative disorders (CMD) (Smith et al., 2008; Lacronique et al., 1997). Although the breakpoints of each translocation are known, characterization of the partner gene has not been done in many of the cases reported due to insufficient sample or other factors. In the present study we review all translocations involving JAK2 that have been reported in the literature.     

Autophagy Plays a Role in Chloroplast Degradation during Senescence in Individually Darkened Leaves

Chloroplasts contain approximately 80% of total leaf nitrogen and represent a major source of recycled nitrogen during leaf senescence. While bulk degradation of the cytosol and organelles in plants is mediated by autophagy, its role in chloroplast catabolism is largely unknown. We investigated the effects of autophagy disruption on the number and size of chloroplasts during senescence. When leaves were individually darkened, senescence was promoted similarly in both wild-type Arabidopsis (Arabidopsis thaliana) and in an autophagy-defective mutant, atg4a4b-1. The number and size of chloroplasts decreased in darkened leaves of wild type, while the number remained constant and the size decrease was suppressed in atg4a4b-1. When leaves of transgenic plants expressing stroma-targeted DsRed were individually darkened, a large accumulation of fluorescence in the vacuolar lumen was observed. Chloroplasts exhibiting chlorophyll fluorescence, as well as Rubisco-containing bodies, were also observed in the vacuole. No accumulation of stroma-targeted DsRed, chloroplasts, or Rubisco-containing bodies was observed in the vacuoles of the autophagy-defective mutant. We have succeeded in demonstrating chloroplast autophagy in living cells and provide direct evidence of chloroplast transportation into the vacuole.Chloroplasts contain 75% to 80% of total leaf nitrogen mainly as proteins (Makino and Osmond, 1991). During leaf senescence, chloroplast proteins are gradually degraded as a major source of nitrogen for new growth (Wittenbach, 1978; Friedrich and Huffaker, 1980; Mae et al., 1984), correlating with a decline in photosynthetic activity, while chloroplasts gradually shrink and transform into gerontoplasts, characterized by the disintegration of the thylakoid membranes and accumulation of plastoglobuli (for a recent review, see Krupinska, 2006). Concomitantly, a decline in the cellular population of chloroplasts is also evident in many cases, for example, during natural (Kura-Hotta et al., 1990; Inada et al., 1998), dark-induced (Wittenbach et al., 1982), and nutrient-limited senescence (Mae et al., 1984; Ono et al., 1995), suggesting the existence of a whole chloroplast degradation system. Some electron microscopic studies have shown whole chloroplasts in the central vacuole, which is rich in lytic hydrolases (Wittenbach et al., 1982; Minamikawa et al., 2001). However, there is no direct evidence of chloroplasts moving into the vacuole in living cells and the mechanism of transport is not yet understood (Hörtensteiner and Feller, 2002; Krupinska, 2006).The most abundant chloroplast protein is Rubisco (EC 4.1.1.39), comprising approximately 50% of the soluble protein (Wittenbach, 1978). The amount of Rubisco decreases rapidly in the early phase of leaf senescence, although more slowly in the later phase (Friedrich and Huffaker, 1980; Mae et al., 1984). In contrast, the chloroplast number remains relatively constant, making it impossible to explain Rubisco loss solely by whole chloroplast degradation. However, the mechanism of intrachloroplastic Rubisco degradation is still unknown (for review, see Feller et al., 2008). Using immunoelectron microscopy, we previously demonstrated in naturally senescing wheat (Triticum aestivum) leaves that Rubisco is released from chloroplasts into the cytoplasm and transported to the vacuole for subsequent degradation in small spherical bodies, named Rubisco-containing bodies (RCBs; Chiba et al., 2003). Similar chloroplast-derived structures were also subsequently confirmed in senescent leaves of soybean (Glycine max) and/or Arabidopsis (Arabidopsis thaliana) by electron microscopy (Otegui et al., 2005), and recently in tobacco (Nicotiana tabacum) leaves by immunoelectron microscopy, although the authors gave them a different name, Rubisco vesicular bodies (Prins et al., 2008). RCBs have double membranes, which seem to be derived from the chloroplast envelope; thus, the RCB-mediated degradation of stromal proteins represents a potential mechanism for chloroplast shrinkage during senescence. We recently demonstrated that Rubisco and stroma-targeted fluorescent proteins can be mobilized to the vacuole by ATG-dependent autophagy via RCBs, using leaves treated with concanamycin A, a vacuolar H⁺-ATPase inhibitor (Ishida et al., 2008). To investigate further, we wished to observe chloroplast autophagy and degradation directly in living cells to determine whether autophagy is responsible for chloroplast shrinkage and whether it is involved in the vacuolar degradation of whole chloroplasts during leaf senescence.Autophagy is known to be a major system for the bulk degradation of intracellular proteins and organelles in the vacuole in yeast and plants, or the lysosome in animals (for detailed mechanisms, see reviews by Ohsumi, 2001; Levine and Klionsky, 2004; Thompson and Vierstra, 2005; Bassham et al., 2006). In those systems, a portion of the cytoplasm, including entire organelles, is engulfed in membrane-bound vesicles and delivered to the vacuole/lysosome. A recent genome-wide search confirmed that Arabidopsis has many genes homologous to the yeast autophagy genes (ATGs; Doelling et al., 2002; Hanaoka et al., 2002; for detailed functions of ATGs, see the reviews noted above). Using knockout mutants of ATGs and a monitoring system with an autophagy marker, GFP-ATG8, numerous studies have demonstrated the presence of the autophagy system in plants and its importance in several biological processes (Yoshimoto et al., 2004; Liu et al., 2005; Suzuki et al., 2005; Thompson et al., 2005; Xiong et al., 2005, 2007; Fujiki et al., 2007; Phillips et al., 2008). These articles suggest that autophagy plays an important role in nutrient recycling during senescence, especially in nutrient-starved plants. The atg mutants exhibited an accelerated loss of some chloroplast proteins, but not all, under nutrient-starved conditions and during senescence, suggesting that autophagy is not the sole mechanism for the degradation of chloroplast proteins; other, as yet unidentified systems must be responsible for the degradation of chloroplast contents when the ATG system is compromised (Levine and Klionsky, 2004; Bassham et al., 2006). However, it still remains likely that autophagy is responsible for the vacuolar degradation of chloroplasts in wild-type plants.Prolonged observation is generally required to follow leaf senescence events in naturally aging leaves and senescence-associated processes tend to become chaotic over time. To observe chloroplast degradation over a short period, and to draw clear conclusions, a suitable experimental model of leaf senescence is required. Weaver and Amasino (2001) reported that senescence is rapidly induced in individually darkened leaves (IDLs) of Arabidopsis, but retarded in plants subjected to full darkness. In addition, Keech et al. (2007) observed a significant decrease of both the number and size of chloroplasts in IDLs within 6 d.In this study, using IDLs as a senescence model, we aimed to investigate the involvement of autophagy in chloroplast degradation. We show direct evidence for the transport of whole chloroplasts and RCBs to the vacuole by autophagy.     

Insect odorant receptors: channeling scent

Odorant detection in insects involves heterodimers between an odorant receptor (OR) and a conserved seven-transmembrane protein called Or83b, but the exact mechanism of OR signal transduction is unclear. Two recent studies in Nature (Sato et al., 2008; Wicher et al., 2008) now reveal that these OR-Or83b heterodimers form odorant-gated ion channels, revealing a surprising new mode of olfactory transduction.     

Rehabilitation of a contract killer: caspase-3 directs stem cell differentiation

Activation of caspase-3 is generally acknowledged as a penultimate step in apoptotic cell death pathways. Two studies in this issue of Cell Stem Cell (Fujita et al., 2008; Janzen et al., 2008) provide compelling data to demonstrate that caspase-3 is also a conserved inductive cue for stem cell differentiation.