Transcript PowerPoint ******

Molecular biology: RNA interference hangs by a thread
Mikel Zaratiegui
The Paf1 protein complex in fission yeast has been found to protect proteincoding genes from inhibition by RNA-mediated silencing of transcription, by
stimulating the release of nascent transcripts from DNA. See Letter p.248
The Paf1 complex represses small-RNA-mediated epigenetic gene silencing
Katarzyna Maria Kowalik, Yukiko Shimada, Valentin Flury, Michael Beda Stadler, Julia Batki & Marc Bühler
RNA interference (RNAi) refers to the ability of exogenously introduced double-stranded
RNA to silence expression of homologous sequences. Silencing is initiated when the enzyme Dicer
processes the double-stranded RNA into small interfering RNAs (siRNAs). Small RNA molecules are
incorporated into Argonaute-protein-containing effector complexes, which they guide to
complementary targets to mediate different types of gene silencing, specifically post-transcriptional
gene silencing and chromatin-dependent gene silencing1. Although endogenous small RNAs have crucial
roles in chromatin-mediated processes across kingdoms, efforts to initiate chromatin modifications in
trans by using siRNAs have been inherently difficult to achieve in all eukaryotic cells. Using fission yeast,
here we show that RNAi-directed heterochromatin formation is negatively controlled by the highly
conserved RNA polymerase-associated factor 1 complex (Paf1C). Temporary expression of a synthetic
hairpin RNA in Paf1C mutants triggers stable heterochromatin formation at homologous loci, effectively
silencing genes in trans. This repressed state is propagated across generations by the continual
production of secondary siRNAs, independently of the synthetic hairpin RNA. Our data support a model
in which Paf1C prevents targeting of nascent transcripts by the siRNA-containing RNA-induced
transcriptional silencing complex and thereby epigenetic gene silencing, by promoting efficient
transcription termination and rapid release of the RNA from the site of transcription. We show that
although compromised transcription termination is sufficient to initiate the formation of bi-stable
heterochromatin by trans-acting siRNAs, impairment of both transcription termination and nascent
transcript release is imperative to confer stability to the repressed state. Our work uncovers a novel
mechanism for small-RNA-mediated epigenome regulation and highlights fundamental roles for Paf1C
and the RNAi machinery in building epigenetic memory.
Metabolism: Growth in the fat lane
Robert A. Egnatchik
& Ralph J. DeBerardinis
Analysis of endothelial cells, which are involved in blood-vessel formation,
unexpectedly reveals that proliferation in this cell type depends on fatty-acid
oxidation to support DNA synthesis. See Article p.192
Fatty acid carbon is essential for dNTP synthesis in endothelial cells.
Schoors S1, Bruning U1, Missiaen R1, Queiroz KC1, Borgers G1, Elia I2, Zecchin A1, Cantelmo
AR1, Christen S2, Goveia J1, Heggermont W3, Goddé L1, Vinckier S1, Van Veldhoven PP4, Eelen
G1, Schoonjans L1, Gerhardt H5, Dewerchin M1, Baes M6, De Bock K7, Ghesquière B1, Lunt SY8,
Fendt SM2, Carmeliet P1.
The metabolism of endothelial cells during vessel sprouting remains poorly studied.
Here we report that endothelial loss of CPT1A, a rate-limiting enzyme of fatty acid oxidation
(FAO), causes vascular sprouting defects due to impaired proliferation, not migration, of human
and murine endothelial cells. Reduction of FAO in endothelial cells did not cause energy
depletion or disturb redox homeostasis, but impaired de novo nucleotide synthesis for DNA
replication. Isotope labelling studies in control endothelial cells showed that fatty acid carbons
substantially replenished the Krebs cycle, and were incorporated into aspartate (a nucleotide
precursor), uridine monophosphate (a precursor of pyrimidine nucleoside triphosphates) and
DNA. CPT1A silencing reduced these processes and depleted endothelial cell stores of aspartate
and deoxyribonucleoside triphosphates. Acetate (metabolized to acetyl-CoA, thereby
substituting for the depleted FAO-derived acetyl-CoA) or a nucleoside mix rescued the
phenotype of CPT1A-silenced endothelial cells. Finally, CPT1 blockade inhibited pathological
ocular angiogenesis in mice, suggesting a novel strategy for blocking angiogenesis.
EZH2 inhibition sensitizes BRG1 and EGFR mutant lung tumours to
TopoII inhibitors.
Fillmore CM1, Xu C2, Desai PT3, Berry JM3, Rowbotham SP1, Lin YJ4, Zhang H2,
Marquez VE5, Hammerman PS6, Wong KK2, Kim CF1.
Non-small-cell lung cancer is the leading cause of cancer-related death
worldwide. Chemotherapies such as the topoisomerase II (TopoII) inhibitor etoposide
effectively reduce disease in a minority of patients with this cancer; therefore,
alternative drug targets, including epigenetic enzymes, are under consideration for
therapeutic intervention. A promising potential epigenetic target is the
methyltransferase EZH2, which in the context of the polycomb repressive complex 2
(PRC2) is well known to tri-methylate histone H3 at lysine 27 (H3K27me3) and elicit
gene silencing. Here we demonstrate that EZH2 inhibition has differential effects on
the TopoII inhibitor response of non-small-cell lung cancers in vitro and in vivo. EGFR
and BRG1 mutations are genetic biomarkers that predict enhanced sensitivity to
TopoII inhibitor in response to EZH2 inhibition. BRG1 loss-of-function mutant
tumours respond to EZH2 inhibition with increased S phase, anaphase bridging,
apoptosis and TopoII inhibitor sensitivity. Conversely, EGFR and BRG1 wild-type
tumours upregulate BRG1 in response to EZH2 inhibition and ultimately become
more resistant to TopoII inhibitor. EGFR gain-of-function mutant tumours are also
sensitive to dual EZH2 inhibition and TopoII inhibitor, because of genetic antagonism
between EGFR and BRG1. These findings suggest an opportunity for precision
Cardiology: Race for healthy hearts
Marc-Phillip Hitz & Gregor Andelfinger
Transplantation experiments in mice reveal that the increased risk of congenital
heart disease in the pups of older mothers is not conferred by ageing eggs, but
by the mothers' age, and can be mitigated by exercise. See Letter p.230
The maternal-age-associated risk of congenital heart disease is
modifiable
Claire E. Schulkey,
Suk D. Regmi,
Rachel A. Magnan,
Megan T.
Danzo, Herman Luther, Alayna K. Hutchinson,
Adam A. Panzer, Mary M. Grady,
David B. Wilson & Patrick Y. Jay
Scientific Reports
10
11
Dopamine Modulates Egalitarian (all people deserve equal rights and opport
unities) Behavior in Humans
Helen Wills Neuroscience Institute, University of California, Berkeley, 175 Li Ka S
hing Center, Berkeley, CA 94720, USA
Summary
Egalitarian motives form a powerful force in promoting prosocial behavior and enabling l
arge-scale cooperation in the human species [ 1 ]. At the neural level, there is substantial,
albeit correlational, evidence suggesting a link between dopamine and such behavior [ 2,
3 ]. However, important questions remain about the specific role of dopamine in setting
or modulating behavioral sensitivity to prosocial concerns. Here, using a combination of
pharmacological tools and economic games, we provide critical evidence for a causal inv
olvement of dopamine in human egalitarian tendencies. Specifically, using the brain pene
trant catechol-O-methyl transferase (COMT) inhibitor tolcapone [ 4, 5 ], we investigated t
he causal relationship between dopaminergic mechanisms and two prosocial concerns at
the core of a number of widely used economic games: (1) the extent to which individuals
directly value the material payoffs of others, i.e., generosity, and (2) the extent to which t
hey are averse to differences between their own payoffs and those of others, i.e., inequity
. We found that dopaminergic augmentation via COMT inhibition increased egalitarian te
ndencies in participants who played an extended version of the dictator game [ 6 ]. Striki
ngly, computational modeling of choice behavior [ 7 ] revealed that tolcapone exerted sel
ective effects on inequity aversion, and not on other computational components such as
the extent to which individuals directly value the material payoffs of others. Together, the
se data shed light on the causal relationship between neurochemical systems and human
prosocial behavior and have potential implications for our understanding of the complex12
Brain Regeneration in Drosophila Involves Comparison of Neuronal Fitness
Institute of Cell Biology (IZB), University of Bern, Bern 3012, Switzerland
Summary
Darwinian-like cell selection has been studied during development and cancer [ 1–11 ]. C
ell selection is often mediated by direct intercellular comparison of cell fitness, using “fitn
ess fingerprints” [ 12–14 ]. In Drosophila, cells compare their fitness via several isoforms o
f the transmembrane protein Flower [ 12, 13 ]. Here, we studied the role of intercellular fi
tness comparisons during regeneration. Regeneration-competent organisms are tradition
ally injured by amputation [ 15, 16 ], whereas in clinically relevant injuries such as local is
chemia or traumatic injury, damaged tissue remains within the organ [ 17–19 ]. We reaso
ned that “Darwinian” interactions between old and newly formed tissues may be importa
nt in the elimination of damaged cells. We used a model of adult brain regeneration in D
rosophila in which mechanical puncture activates regenerative neurogenesis based on da
mage-responsive stem cells [ 20 ]. We found that apoptosis after brain injury occurs in d
amage-exposed tissue located adjacent to zones of de novo neurogenesis. Injury-affecte
d neurons start to express isoforms of the Flower cell fitness indicator protein not found
on intact neurons. We show that this change in the neuronal fitness fingerprint is require
d to recognize and eliminate such neurons. Moreover, apoptosis is inhibited if all neurons
express “low-fitness” markers, showing that the availability of new and healthy cells drives
tissue replacement. In summary, we found that elimination of impaired tissue during brai
n regeneration requires comparison of neuronal fitness and that tissue replacement after
brain damage is coordinated by injury-modulated fitness fingerprints. Intercellular fitness
comparisons between old and newly formed tissues could be a general mechanism of re
13
generative tissue replacement.
Nature reviews Mol. Cell Biol.