251 research outputs found
Habitable Zones in the Universe
Habitability varies dramatically with location and time in the universe. This
was recognized centuries ago, but it was only in the last few decades that
astronomers began to systematize the study of habitability. The introduction of
the concept of the habitable zone was key to progress in this area. The
habitable zone concept was first applied to the space around a star, now called
the Circumstellar Habitable Zone. Recently, other, vastly broader, habitable
zones have been proposed. We review the historical development of the concept
of habitable zones and the present state of the research. We also suggest ways
to make progress on each of the habitable zones and to unify them into a single
concept encompassing the entire universe.Comment: 71 pages, 3 figures, 1 table; to be published in Origins of Life and
Evolution of Biospheres; table slightly revise
The Caenorhabditis elegans GATA Factor ELT-1 Works through the Cell Proliferation Regulator BRO-1 and the Fusogen EFF-1 to Maintain the Seam Stem-Like Fate
Seam cells in Caenorhabditis elegans provide a paradigm for the stem cell mode of division, with the ability to both self-renew and produce daughters that differentiate. The transcription factor RNT-1 and its DNA binding partner BRO-1 (homologues of the mammalian cancer-associated stem cell regulators RUNX and CBFβ, respectively) are known rate-limiting regulators of seam cell proliferation. Here, we show, using a combination of comparative genomics and DNA binding assays, that bro-1 expression is directly regulated by the GATA factor ELT-1. elt-1(RNAi) animals display similar seam cell lineage defects to bro-1 mutants, but have an additional phenotype in which seam cells lose their stem cell-like properties and differentiate inappropriately by fusing with the hyp7 epidermal syncytium. This phenotype is dependent on the fusogen EFF-1, which we show is repressed by ELT-1 in seam cells. Overall, our data suggest that ELT-1 has dual roles in the stem-like seam cells, acting both to promote proliferation and prevent differentiation
PP1γ2 and PPP1R11 Are Parts of a Multimeric Complex in Developing Testicular Germ Cells in which their Steady State Levels Are Reciprocally Related
Mice lacking the protein phosphatase 1 gamma isoforms, PP1γ1 and PP1γ2, are male-sterile due to defective germ cell morphogenesis and apoptosis. However, this deficiency causes no obvious abnormality in other tissues. A biochemical approach was employed to learn how expression versus deficiency of PP1γ2, the predominant PP1 isoform in male germ cells, affects spermatogenesis. Methods used in this study include column chromatography, western blot and northern blot analyses, GST pull-down assays, immunoprecipitation, non-denaturing gel electrophoresis, phosphatase enzyme assays, protein sequencing, and immunohistochemistry. We report for the first time that in wild-type testis, PP1γ2 forms an inactive complex with actin, protein phosphatase 1 regulatory subunit 7 (PPP1R7), and protein phosphatase 1 regulatory subunit 11 (PPP1R11), the latter, a potent PP1 inhibitor. Interestingly, PPP1R11 protein, but not its mRNA level, falls significantly in PP1γ-null testis where mature sperm are virtually absent. Conversely, both mature sperm numbers and the PPP1R11 level increase substantially in PP1γ-null testis expressing transgenic PP1γ2. PPP1R11 also appears to be ubiquitinated in PP1γ-null testis. The levels of PP1γ2 and PPP1R11 were increased in phenotypically normal PP1α-null testis. However, in PP1α-null spleen, where PP1γ2 normally is not expressed, PPP1R11 levels remained unchanged. Our data clearly show a direct reciprocal relationship between the levels of the protein phosphatase isoform PP1γ2 and its regulator PPP1R11, and suggest that complex formation between these polypeptides in testis may prevent proteolysis of PPP1R11 and thus, germ cell apoptosis
LOFAR Detection of 110-188 MHz Emission and Frequency-Dependent Activity from FRB 20180916B
FRB 20180916B is a well-studied repeating fast radio burst source. Its
proximity (~150 Mpc), along with detailed studies of the bursts, have revealed
many clues about its nature -- including a 16.3-day periodicity in its
activity. Here we report on the detection of 18 bursts using LOFAR at 110-188
MHz, by far the lowest-frequency detections of any FRB to date. Some bursts are
seen down to the lowest-observed frequency of 110 MHz, suggesting that their
spectra extend even lower. These observations provide an order-of-magnitude
stronger constraint on the optical depth due to free-free absorption in the
source's local environment. The absence of circular polarization and nearly
flat polarization angle curves are consistent with burst properties seen at
300-1700 MHz. Compared with higher frequencies, the larger burst widths
(~40-160 ms at 150 MHz) and lower linear polarization fractions are likely due
to scattering. We find ~2-3 rad/m^2 variations in the Faraday rotation measure
that may be correlated with the activity cycle of the source. We compare the
LOFAR burst arrival times to those of 38 previously published and 22 newly
detected bursts from the uGMRT (200-450 MHz) and CHIME/FRB (400-800 MHz).
Simultaneous observations show 5 CHIME/FRB bursts when no emission is detected
by LOFAR. We find that the burst activity is systematically delayed towards
lower frequencies by ~3 days from 600 MHz to 150 MHz. We discuss these results
in the context of a model in which FRB 20180916B is an interacting binary
system featuring a neutron star and high-mass stellar companion.Comment: Accepted for publication by ApJ
Runx Expression Is Mitogenic and Mutually Linked to Wnt Activity in Blastula-Stage Sea Urchin Embryos
The Runt homology domain (Runx) defines a metazoan family of sequence-specific transcriptional regulatory proteins that are critical for animal development and causally associated with a variety of mammalian cancers. The sea urchin Runx gene SpRunt-1 is expressed throughout the blastula stage embryo, and is required globally during embryogenesis for cell survival and differentiation.Depletion of SpRunt-1 by morpholino antisense-mediated knockdown causes a blastula stage deficit in cell proliferation, as shown by bromodeoxyuridine (BrdU) incorporation and direct cell counts. Reverse transcription coupled polymerase chain reaction (RT-PCR) studies show that the cell proliferation deficit is presaged by a deficit in the expression of several zygotic wnt genes, including wnt8, a key regulator of endomesoderm development. In addition, SpRunt-1-depleted blastulae underexpress cyclinD, an effector of mitogenic Wnt signaling. Blastula stage cell proliferation is also impeded by knockdown of either wnt8 or cyclinD. Chromatin immunoprecipitation (ChIP) indicates that Runx target sites within 5′ sequences flanking cyclinD, wnt6 and wnt8 are directly bound by SpRunt-1 protein at late blastula stage. Furthermore, experiments using a green fluorescent protein (GFP) reporter transgene show that the blastula-stage operation of a cis-regulatory module previously shown to be required for wnt8 expression (Minokawa et al., Dev. Biol. 288: 545–558, 2005) is dependent on its direct sequence-specific interaction with SpRunt-1. Finally, inhibitor studies and immunoblot analysis show that SpRunt-1 protein levels are negatively regulated by glycogen synthase kinase (GSK)-3.These results suggest that Runx expression and Wnt signaling are mutually linked in a feedback circuit that controls cell proliferation during development
Periodic activity from a fast radio burst source
Fast radio bursts (FRBs) are bright, millisecond-duration radio transients
originating from extragalactic distances. Their origin is unknown. Some FRB
sources emit repeat bursts, ruling out cataclysmic origins for those events.
Despite searches for periodicity in repeat burst arrival times on time scales
from milliseconds to many days, these bursts have hitherto been observed to
appear sporadically, and though clustered, without a regular pattern. Here we
report the detection of a day periodicity (or possibly a
higher-frequency alias of that periodicity) from a repeating FRB
180916.J0158+65 detected by the Canadian Hydrogen Intensity Mapping Experiment
Fast Radio Burst Project (CHIME/FRB). In 38 bursts recorded from September
16th, 2018 through February 4th, 2020, we find that all bursts arrive in a
5-day phase window, and 50% of the bursts arrive in a 0.6-day phase window. Our
results suggest a mechanism for periodic modulation either of the burst
emission itself, or through external amplification or absorption, and disfavour
models invoking purely sporadic processes
Identification of Saturn's magnetospheric regions and associated plasma processes: Synopsis of Cassini observations during orbit insertion
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94634/1/rog1672.pd
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