Monday, October 10, 2011

Hot off the presses! Oct 11 Curr Biol

The Oct 11 issue of the Curr Biol is now up on Pubget (About Curr Biol): if you're at a subscribing institution, just click the link in the latest link at the home page. (Note you'll only be able to get all the PDFs in the issue if your institution subscribes to Pubget.)

Latest Articles Include:

  • Peroxisomes
    - Curr Biol 21(19):R800-R801 (2011)
  • Females floated first in bubble-rafting snails
    - Curr Biol 21(19):R802-R803 (2011)
    Ever since Mivart asked Darwin to explain a bird's use for half a wing, biologists have been challenged to explain extraordinary evolutionary change mechanistically. Here, we investigate the enigmatic evolutionary origins of Janthinidae, a family of marine snails that raft passively in the neuston, a vast oceanic surface habitat, by constructing floats of mucus bubbles. We present the first molecular phylogeny including Janthinidae, which confirms that janthinids are derived from Epitoniidae (wentletraps) — benthic predators and parasites of sea anemones and corals. Our data support the hypothesis that floats and rafting evolved via modified epitoniid egg masses rather than by juvenile droguing. Our phylogeny also reveals sequential modifications of float formation and function among janthinid lineages. We interpret these changes as sequential adaptations to a neustonic existence, a conclusion supported by the positive association of derived janthinid traits with eco! logical prevalence.
  • Novel Ecosystems: Altering Fish Assemblages in Warming Waters
    - Curr Biol 21(19):R822-R824 (2011)
    The effects of increasing sea temperatures extend far beyond changes in species' distributions. By altering local fish abundances, temperature changes will have profound effects on the structure, functioning and services of marine ecosystems.
  • Bulk Cytoplasmic Actin and Its Functions in Meiosis and Mitosis
    - Curr Biol 21(19):R825-R830 (2011)
    Discussions of actin cell biology generally focus on the cortex, a thin, actin-rich layer of cytoplasm under the plasma membrane. Here we review the much less studied biology of actin filaments deeper in the cytoplasm and their recently revealed functions in mitosis and meiosis that are most prominent in large oocyte, egg and early embryo cells. The cellular functions of cytoplasmic actin range from the assembly and positioning of meiotic spindles to the prevention of cytoplasmic streaming. We discuss the possible use of evolutionarily conserved mechanisms to nucleate and organize actin filaments to achieve these diverse cellular functions, the cell-cycle regulation of these functions, and the many unanswered questions about this largely unexplored mechanism of cytoplasmic organization.
  • In Vivo Optogenetic Stimulation of Neocortical Excitatory Neurons Drives Brain-State-Dependent Inhibition
    - Curr Biol 21(19):1593-1602 (2011)
    Background Synaptic interactions between excitatory and inhibitory neocortical neurons are important for mammalian sensory perception. Synaptic transmission between identified neurons within neocortical microcircuits has mainly been studied in brain slice preparations in vitro. Here, we investigate brain-state-dependent neocortical synaptic interactions in vivo by combining the specificity of optogenetic stimulation with the precision of whole-cell recordings from postsynaptic excitatory glutamatergic neurons and GFP-labeled inhibitory GABAergic neurons targeted through two-photon microscopy. Results Channelrhodopsin-2 (ChR2) stimulation of excitatory layer 2/3 barrel cortex neurons evoked larger and faster depolarizing postsynaptic potentials and more synaptically driven action potentials in fast-spiking (FS) GABAergic neurons compared to both non-fast-spiking (NFS) GABAergic neurons and postsynaptic excitatory pyramidal neurons located within the same neocortical microcircuit. The number of action potentials evoked in ChR2-expressing neurons showed low trial-to-trial variability, but postsynaptic responses varied strongly with near-linear dependence upon spontaneously driven changes in prestimulus membrane potential. Postsynaptic responses in excitatory neurons had reversal potentials, which were hyperpolarized relative to action potential threshold and were therefore inhibitory. Reversal potentials measured in postsynaptic GABAergic neurons were close to action potential threshold. Postsynaptic inhibitory neurons preferentially fired synaptically driven action potenti! als from spontaneously depolarized network states, with stronger state-dependent modulation in NFS GABAergic neurons compared to FS GABAergic neurons. Conclusions Inhibitory neurons appear to dominate neocortical microcircuit function, receiving stronger local excitatory synaptic input and firing more action potentials compared to excitatory neurons. In mouse layer 2/3 barrel cortex, we propose that strong state-dependent recruitment of inhibitory neurons drives competition among excitatory neurons enforcing sparse coding.
  • An EDMD Mutation in C. elegans Lamin Blocks Muscle-Specific Gene Relocation and Compromises Muscle Integrity
    - Curr Biol 21(19):1603-1614 (2011)
    Background In worms, as in other organisms, many tissue-specific promoters are sequestered at the nuclear periphery when repressed and shift inward when activated. It has remained unresolved, however, whether the association of facultative heterochromatin with the nuclear periphery, or its release, has functional relevance for cell or tissue integrity. Results Using ablation of the unique lamin gene in C. elegans, we show that lamin is necessary for the perinuclear positioning of heterochromatin. We then express at low levels in otherwise wild-type worms a lamin carrying a point mutation, Y59C, which in humans is linked to an autosomal-dominant form of Emery-Dreifuss muscular dystrophy. Using embryos and differentiated tissues, we track the subnuclear position of integrated heterochromatic arrays and their expression. In LMN-1 Y59C-expressing worms, we see abnormal retention at the nuclear envelope of a gene array bearing a muscle-specific promoter. This correlates with impaired activation of the array-borne myo-3 promoter and altered expression of a number of muscle-specific genes. However, an equivalent array carrying the intestine-specific pha-4 promoter is expressed normally and shifts inward when activated in gut cells of LMN-1 Y59C worms. Remarkably, adult LMN-1 Y59C animals have selectively perturbed body muscle ultrastruct! ure and reduced muscle function. Conclusion Lamin helps sequester heterochromatin at the nuclear envelope, and wild-type lamin permits promoter release following tissue-specific activation. A disease-linked point mutation in lamin impairs muscle-specific reorganization of a heterochromatic array during tissue-specific promoter activation in a dominant manner. This dominance and the correlated muscle dysfunction in LMN-1 Y59C worms phenocopies Emery-Dreifuss muscular dystrophy.
  • Parallel Processing of Appetitive Short- and Long-Term Memories In Drosophila
    - Curr Biol 21(19):1647-1653 (2011)
    It is broadly accepted that long-term memory (LTM) is formed sequentially after learning and short-term memory (STM) formation, but the nature of the relationship between early and late memory traces remains heavily debated [ [1] , [2] , [3] , [4] and [5] ]. To shed light on this issue, we used an olfactory appetitive conditioning in Drosophila, wherein starved flies learned to associate an odor with the presence of sugar [6]. We took advantage of the fact that both STM and LTM are generated after a unique conditioning cycle [ [7] and [8] ] to demonstrate that appetitive LTM is able to form independently of STM. More specifically, we show that (1) STM retrieval involves output from ホウ neurons of the mushroom body (MB), i.e., the olfactory memory center [ [9] and [10] ], whereas LTM retrieval involves output from ホアホイ MB neurons; (2) STM information is not transferred from ホウ neurons to ホアホイ neurons for LTM formation; and (3) the adenyly! l cyclase RUT, which is thought to operate as a coincidence detector between the olfactory stimulus and the sugar stimulus [ [11] , [12] , [13] and [14] ], is required independently in ホウ neurons to form appetitive STM and in ホアホイ neurons to form LTM. Taken together, these results demonstrate that appetitive short- and long-term memories are formed and processed in parallel.
  • Single-Unit Responses Selective for Whole Faces in the Human Amygdala
    - Curr Biol 21(19):1654-1660 (2011)
    The human amygdala is critical for social cognition from faces, as borne out by impairments in recognizing facial emotion following amygdala lesions [1] and differential activation of the amygdala by faces [ [2] , [3] , [4] and [5] ]. Single-unit recordings in the primate amygdala have documented responses selective for faces, their identity, or emotional expression [ [6] and [7] ], yet how the amygdala represents face information remains unknown. Does it encode specific features of faces that are particularly critical for recognizing emotions (such as the eyes), or does it encode the whole face, a level of representation that might be the proximal substrate for subsequent social cognition? We investigated this question by recording from over 200 single neurons in the amygdalae of seven neurosurgical patients with implanted depth electrodes [8]. We found that approximately half of all neurons responded to faces or parts of faces. Approximately 20% of all neurons ! responded selectively only to the whole face. Although responding most to whole faces, these neurons paradoxically responded more when only a small part of the face was shown compared to when almost the entire face was shown. We suggest that the human amygdala plays a predominant role in representing global information about faces, possibly achieved through inhibition between individual facial features.
  • Practicing Coarse Orientation Discrimination Improves Orientation Signals in Macaque Cortical Area V4
    - Curr Biol 21(19):1661-1666 (2011)
    Practice improves the performance in visual tasks, but mechanisms underlying this adult brain plasticity are unclear. Single-cell studies reported no [1], weak [2], or moderate [ [3] and [4] ] perceptual learning-related changes in macaque visual areas V1 and V4, whereas none were found in middle temporal (MT) [5]. These conflicting results and modeling of human (e.g., [ [6] and [7] ]) and monkey data [8] suggested that changes in the readout of visual cortical signals underlie perceptual learning, rather than changes in these signals. In the V4 learning studies, monkeys discriminated small differences in orientation, whereas in the MT study, the animals discriminated opponent motion directions. Analogous to the latter study, we trained monkeys to discriminate static orthogonal orientations masked by noise. V4 neurons showed robust increases in their capacity to discriminate the trained orientations during the course of the training. This effect was observed during! discrimination and passive fixation but specifically for the trained orientations. The improvement in neural discrimination was due to decreased response variability and an increase of the difference between the mean responses for the two trained orientations. These findings demonstrate that perceptual learning in a coarse discrimination task indeed can change the response properties of a cortical sensory area.
  • Decoding Successive Computational Stages of Saliency Processing
    - Curr Biol 21(19):1667-1671 (2011)
    An important requirement for vision is to identify interesting and relevant regions of the environment for further processing. Some models assume that salient locations from a visual scene are encoded in a dedicated spatial saliency map [ [1] and [2] ]. Then, a winner-take-all (WTA) mechanism [ [1] and [2] ] is often believed to threshold the graded saliency representation and identify the most salient position in the visual field. Here we aimed to assess whether neural representations of graded saliency and the subsequent WTA mechanism can be dissociated. We presented images of natural scenes while subjects were in a scanner performing a demanding fixation task, and thus their attention was directed away. Signals in early visual cortex and posterior intraparietal sulcus (IPS) correlated with graded saliency as defined by a computational saliency model. Multivariate pattern classification [ [3] and [4] ] revealed that the most salient position in the visual field! was encoded in anterior IPS and frontal eye fields (FEF), thus reflecting a potential WTA stage. Our results thus confirm that graded saliency and WTA-thresholded saliency are encoded in distinct neural structures. This could provide the neural representation required for rapid and automatic orientation toward salient events in natural environments.
  • Mobile 24 nt Small RNAs Direct Transcriptional Gene Silencing in the Root Meristems of Arabidopsis thaliana
    - Curr Biol 21(19):1678-1683 (2011)
    RNA silencing in flowering plants generates a signal that moves between cells and through the phloem [ [1] and [2] ]. Nucleotide sequence specificity of the signal is conferred by 21, 22, and 24 nucleotide (nt) sRNAs that are generated by Dicer-like (DCL) proteins [3]. In the recipient cells these sRNAs bind to Argonaute (AGO) effectors of silencing and the 21 nt sRNAs mediate posttranscriptional regulation (PTGS) via mRNA cleavage [4] whereas the 24 nt sRNAs are associated with RNA-dependent DNA methylation (RdDM) [5] that may underlie transcriptional gene silencing (TGS). Intriguingly, genes involved in TGS are required for graft-transmissible gene silencing associated with PTGS [6]. However, some of the same genes were also required for spread of a PTGS silencing signal out of the veins of Arabidopsis [7], and grafting tests failed to demonstrate direct transmission of TGS signals [ [8] , [9] and [10] ]. It seemed likely, therefore, that mobile silencing is ass! ociated only with PTGS. To address this possibility, we grafted TGS-inducing wild-type Arabidopsis and a mutant that is compromised in 24 nt sRNA production onto a wild-type reporter line. The 21–24 nt sRNAs from the TGS construct were transmitted across a graft union but only the 24 nt sRNAs directed RdDM and TGS of a transgene promoter in meristematic cells. These data extend the significance of an RNA silencing signal to embrace epigenetics and transcriptional gene silencing and support the hypothesis that these signals transmit information to meristematic cells where they initiate persistent epigenetic changes that may influence growth, development, and heritable phenotypes.
  • Sensory Neurophysiology: Motion Vision during Motor Action
    - Curr Biol 21(19):1684 (2011)
  • Subcellular Positioning: Unstable Filaments On the Move
    - Curr Biol 21(19):1684 (2011)

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