591 research outputs found
Bioactive ether lipids: primordial modulators of cellular signaling
The primacy of lipids as essential components of cellular membranes is conserved across taxonomic domains. In addition to this crucial role as a semi-permeable barrier, lipids are also increasingly recognized as important signaling molecules with diverse functional mechanisms ranging from cell surface receptor binding to the intracellular regulation of enzymatic cascades. In this review, we focus on ether lipids, an ancient family of lipids having ether-linked structures that chemically differ from their more prevalent acyl relatives. In particular, we examine ether lipid biosynthesis in the peroxisome of mammalian cells, the roles of selected glycerolipids and glycerophospholipids in signal transduction in both prokaryotes and eukaryotes, and finally, the potential therapeutic contributions of synthetic ether lipids to the treatment of cancer
Unravelling the microphysics of polar mesospheric cloud formation
Polar mesospheric clouds are the highest water ice clouds
occurring in the terrestrial atmosphere. They form in the polar summer
mesopause, the coldest region in the atmosphere. It has long been assumed
that these clouds form by heterogeneous nucleation on meteoric smoke
particles which are the remnants of material ablated from meteoroids in the
upper atmosphere. However, until now little was known about the properties
of these nanometre-sized particles and application of the classical theory for
heterogeneous ice nucleation was impacted by large uncertainties. In this
work, we performed laboratory measurements on the heterogeneous ice
formation process at mesopause conditions on small (r=1 to 3 nm)
iron silicate nanoparticles serving as meteoric smoke analogues. We observe
that ice growth on these particles sets in for saturation ratios with
respect to hexagonal ice below Sh=50, a value that is commonly
exceeded during the polar mesospheric cloud season, affirming meteoric smoke
particles as likely nuclei for heterogeneous ice formation in mesospheric
clouds. We present a simple ice-activation model based on the Kelvin–Thomson equation that takes into account the water coverage of iron silicates of
various compositions. The activation model reproduces the experimental data
very well using bulk properties of compact amorphous solid water. This is in
line with the finding from our previous study that ice formation on
iron silicate nanoparticles occurs by condensation of amorphous solid water
rather than by nucleation of crystalline ice at mesopause conditions. Using
the activation model, we also show that for iron silicate particles with dry
radius larger than r=0.6 nm the nanoparticle charge has no significant
effect on the ice-activation threshold.</p
New cloud chamber experiments on the heterogeneous ice nucleation ability of oxalic acid in the immersion mode
The heterogeneous ice nucleation ability of oxalic acid in the immersion
mode has been investigated by controlled expansion cooling runs with
airborne, ternary solution droplets composed of, (i), sodium chloride,
oxalic acid, and water (NaCl/OA/H<sub>2</sub>O) and, (ii), sulphuric acid, oxalic
acid, and water (H<sub>2</sub>SO<sub>4</sub>/OA/H<sub>2</sub>O). Polydisperse aerosol
populations with median diameters ranging from 0.5–0.7 μm and
varying solute concentrations were prepared. The expansion experiments were
conducted in the AIDA aerosol and cloud chamber of the Karlsruhe Institute
of Technology at initial temperatures of 244 and 235 K. In the ternary
NaCl/OA/H<sub>2</sub>O system, solid inclusions of oxalic acid, presumably
nucleated as oxalic acid dihydrate, were formed by temporarily exposing the
ternary solution droplets to a relative humidity below the efflorescence
point of NaCl. The matrix of the crystallised NaCl particulates triggered
the precipitation of the organic crystals which later remained as solid
inclusions in the solution droplets when the relative humidity was
subsequently raised above the deliquescence point of NaCl. The embedded
oxalic acid crystals reduced the critical ice saturation ratio required for
the homogeneous freezing of pure NaCl/H<sub>2</sub>O solution droplets at a
temperature of around 231 K from 1.38 to about 1.32. Aqueous solution
droplets with OA inclusions larger than about 0.27 μm in diameter
efficiently nucleated ice by condensation freezing when they were activated
to micron-sized cloud droplets at 241 K, i.e., they froze well above the
homogeneous freezing temperature of pure water droplets of about 237 K. Our
results on the immersion freezing potential of oxalic acid corroborate the
findings from a recent study with emulsified aqueous solutions containing
crystalline oxalic acid. In those experiments, the crystallisation of oxalic
acid diyhdrate was triggered by a preceding homogeneous freezing cycle with
the emulsion samples. The expansion cooling cycles with ternary
H<sub>2</sub>SO<sub>4</sub>/OA/H<sub>2</sub>O solution droplets were aimed to analyse whether
those findings can be transferred to ice nucleation experiments with
airborne oxalic acid containing aerosol particles. Under our experimental
conditions, the efficiency by which the surface of homogeneously nucleated
ice crystals triggered the precipitation of oxalic acid dihydrate was very
low, i.e., less than one out of a hundred ice crystals that were formed by
homogeneous freezing in a first expansion cooling cycle left behind an
ice-active organic crystal that acted as immersion freezing nucleus in a
second expansion cooling cycle
Essential role of CIB1 in regulating PAK1 activation and cell migration
p21-activated kinases (PAKs) regulate many cellular processes, including cytoskeletal rearrangement and cell migration. In this study, we report a direct and specific interaction of PAK1 with a 22-kD Ca2+-binding protein, CIB1, which results in PAK1 activation both in vitro and in vivo. CIB1 binds to PAK1 within discrete regions surrounding the inhibitory switch domain in a calcium-dependent manner, providing a potential mechanism of CIB1-induced PAK1 activation. CIB1 overexpression significantly decreases cell migration on fibronectin as a result of a PAK1-and LIM kinase–dependent increase in cofilin phosphorylation. Conversely, the RNA interference–mediated depletion of CIB1 increases cell migration and reduces normal adhesion-induced PAK1 activation and cofilin phosphorylation. Together, these results demonstrate that endogenous CIB1 is required for regulated adhesion-induced PAK1 activation and preferentially induces a PAK1-dependent pathway that can negatively regulate cell migration. These results point to CIB1 as a key regulator of PAK1 activation and signaling
In situ characterization of mixed phase clouds using the Small Ice Detector and the Particle Phase Discriminator
Mixed phase clouds (MPCs) represent a great source of uncertainty for both climate predictions and weather forecasts. In particular, there is still a lack of understanding on how ice forms in these clouds. In this work we present a technique to analyze in situ measurements of MPCs performed with the latest instruments from the Small Ice Detector family. These instruments record high-resolution scattering patterns of individual small cloud particles. For the analysis of the scattering patterns we developed an algorithm that can discriminate the phase of the cloud particles. In the case of a droplet, a Mie solution is fitted to the recorded pattern and the size of the corresponding particle is obtained, which allows for a size calibration of the instrument. In the case of an ice particle, its shape is deduced from the scattering pattern.
We apply our data analysis method to measurements from three distinct MPC types. The results from laboratory measurements demonstrate that our technique can discriminate between droplets and ice particles in the same optical size range. This ability was verified by measurements at a mountain top station where we found an alternation of liquid- and ice-dominated cloud regions. The analysis of results from aircraft-based measurements illustrates the ice detection threshold of the technique
In situ characterization of mixed phase clouds using the Small Ice Detector and the Particle Phase Discriminator
Mixed phase clouds (MPCs) represent a great source of uncertainty for both climate predictions and weather forecasts. In particular, there is still a lack of understanding on how ice forms in these clouds. In this work we present a technique to analyze in situ measurements of MPCs performed with the latest instruments from the Small Ice Detector family. These instruments record high resolution scattering patterns of individual small cloud particles. For the analysis of the scattering patterns we developed an algorithm that can discriminate the phase of the cloud particles. In the case of a droplet, a Mie solution is fitted to the recorded pattern and the size of the corresponding particle is obtained, which allows for a size calibration of the instrument. In the case of an ice particle, its shape and roughness are deduced from the scattering pattern. We apply our data analysis method to measurements from three distinct MPC types. The results from laboratory measurements demonstrate that our technique can discriminate between droplets and ice particles in the same optical size range. This ability was verified by measurements at a mountain top station where we found an alternation of liquid and ice dominated cloud regions. The analysis of results from aircraft based measurements illustrates the ice detection threshold of the technique
Range expansion with mutation and selection: dynamical phase transition in a two-species Eden model
The colonization of unoccupied territory by invading species, known as range expansion, is a spatially heterogeneous non-equilibrium growth process. We introduce a two-species Eden growth model to analyze the interplay between uni-directional (irreversible) mutations and selection at the expanding front. While the evolutionary dynamics leads to coalescence of both wild-type and mutant clusters, the non-homogeneous advance of the colony results in a rough front. We show that roughening and domain dynamics are strongly coupled, resulting in qualitatively altered bulk and front properties. For beneficial mutations the front is quickly taken over by mutants and growth proceeds Eden-like. In contrast, if mutants grow slower than wild-types, there is an antagonism between selection pressure against mutants and growth by the merging of mutant domains with an ensuing absorbing state phase transition to an all-mutant front. We find that surface roughening has a marked effect on the critical properties of the absorbing state phase transition. While reference models, which keep the expanding front flat, exhibit directed percolation critical behavior, the exponents of the two-species Eden model strongly deviate from it. In turn, the mutation-selection process induces an increased surface roughness with exponents distinct from that of the classical Eden model
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