Hot off the presses! May 24 Curr Bio

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Latest Articles Include:

• Energy U-turn in Germany
  - Curr Bio 21(10):R379-R381 (2011)
  Following the disaster at Fukushima-Daiichi, chancellor Angela Merkel decided to speed up rather than slow down Germany's exit from nuclear power. Michael Gross reports.
• Lymphoid tissue inducer cells
  - Curr Bio 21(10):R381-R382 (2011)
  
• Isabelle Côté
  - Curr Bio 21(10):R383 (2011)
  
• Septins
  - Curr Bio 21(10):R384-R387 (2011)
  Septins are a family of proteins highly conserved in eukaryotes that have been under intense scrutiny lately because they appear to comprise a novel component of the cytoskeleton. As such, septins play important roles in many cellular processes by providing rigidity to the cell membrane, serving as scaffolds to recruit proteins to specific subcellular locales, and creating membrane diffusion barriers to establish discrete cellular domains. Septins were first discovered in yeast during screens for genes that are critical for cell division. Orthologs have since been identified in animals, fungi, and algae; but, interestingly, septins are absent from higher plants. The number of septin genes ranges from a minimum of two in Caenorhabditis elegans (UNC-59 and UNC-61) to 14 in humans (SEPT1 through SEPT14). To complicate matters, the transcripts of many mammalian septin genes undergo alternative splicing, making the number of unique septin isoforms even greater. Despite this complexity, all septins bind guanosine-5′-triphosphate (GTP), and interact with each other to form complexes. Septin complexes further associate with each other to form large filamentous structures, as is observed with other cytoskeletal proteins. In this primer, we provide an overview of the structural and functional features of septins, and highlight their involvement in various human diseases.
• Cell Size: Chromosomes Get Slapped by a Midzone Ruler
  - Curr Bio 21(10):R388-R390 (2011)
  Spatial and temporal coordination of mitotic events has been generally attributed to the coincidental outcome of increasing cyclin-dependent kinase activity. A recent study reports that mitotic events and structures previously considered to be independently controlled are capable of trans-regulation to ensure genomic integrity.
• Perceptual Learning: Visual Function Improved by LTP/LTD-like Stimulation
  - Curr Bio 21(10):R390-R391 (2011)
  A new behavioral training approach has been found significantly to improve visual function; the results further attest to the high degree of plasticity in sensory systems.
• Chloroplast Signaling: Retrograde Regulation Revelations
  - Curr Bio 21(10):R391-R393 (2011)
  Developing chloroplasts are able to communicate their status to the nucleus and regulate expression of genes whose products are needed for photosynthesis. Heme is revealed to be a signaling molecule for this retrograde communication.
• Memory Formation: Filling in the Gaps in Flies
  - Curr Bio 21(10):R394-R395 (2011)
  Research in Drosophila has many advantages for the study of complex behavior. Two studies identify a new role for chemical and electrical signaling in the anterior paired lateral neurons during memory formation.
• Organelle Dynamics: A Tale of Fusing Nucleoli
  - Curr Bio 21(10):R395-R397 (2011)
  Recent experiments on nucleoli suggest that their dynamic behavior is liquid-like with common fusion events and that the surrounding actin plays an active role in these dynamics.
• Notch Signaling: A Role in Sleep and Stress
  - Curr Bio 21(10):R397-R398 (2011)
  The molecular pathways regulating sleep remain poorly understood. Studies in this issue demonstrate a role for Notch signaling in sleep regulation as well as stress response in both Caenorhabditis elegans and Drosophila.
• Evolutionary Genetics: Evolution with Foresight
  - Curr Bio 21(10):R398-R400 (2011)
  Evolution has no foresight, but produces ad hoc solutions by tinkering with available variation. A new study demonstrates how evolution nevertheless prepares organisms for the future by increasing their evolvability.
• Circadian Rhythms: Lost in Post-Translation
  - Curr Bio 21(10):R400-R402 (2011)
  Multiple studies question the necessity of transcription/translation feedback loops for the generation of circadian rhythms. New data emphasize the necessity of proteasomal degradation for circadian rhythmicity in transcriptionally competent cells.
• Synaptic Growth: Dancing with Adducin
  - Curr Bio 21(10):R402-R405 (2011)
  Manipulations of the actin-capping protein adducin in Drosophila and mammalian neurons provide new insights into the mechanisms linking structural changes to synaptic plasticity and learning. Adducin regulates synaptic remodeling, providing a molecular switch that controls synaptic growth versus disassembly during plasticity.
• Sustained Elongation of Sperm Tail Promoted by Local Remodeling of Giant Mitochondria in Drosophila
  - Curr Bio 21(10):805-814 (2011)
  Background Sperm length in Drosophilidae varies from a few hundred microns to 6 cm as a result of evolutionary selection. In postcopulatory competition, longer sperm have an advantage in positioning their head closer to the egg. Sperm cell elongation can proceed in the absence of an axoneme, suggesting that a mechanism besides intraflagellar transport emerged to sustain it. Results Here we report that sperm elongation in Drosophila melanogaster is driven by the interdependent extension of giant mitochondria and microtubule array that is formed around the mitochondrial surface. In primary cultures of elongating spermatids, we demonstrated that the mitochondrial integrity and local dynamics of microtubules at the tail tip region are essential for uniaxial elongation of the sperm tail. Mitochondria-microtubule linker protein Milton accumulated on mitochondria near the tail tip and is required for the sliding movement of microtubules. Disruption of Milton and its associated protein dMiro, and of potential microtubule crosslinkers Nebbish and Fascetto, caused strong elongation defects, indicating that mitochondria-microtubule association and microtubule crosslinking are required for spermatid tail elongation. Conclusions Mitochondria play unexpected roles in sperm tail elongation in Drosophila by providing a structural platform for microtubule reorganization to support the robust elongation taking place at the tip of the very long sperm tail. The identification of mitochondria as an organizer of cytoskeletal dynamics extends our understanding of mechanisms of cell morphogenesis.
• KIF4 Regulates Midzone Length during Cytokinesis
  - Curr Bio 21(10):815-824 (2011)
  Background Midzones, also called central spindles, are an array of antiparallel microtubules that form during cytokinesis between the separated chromosomes. Midzones can be considered to be platforms that recruit specific proteins and orchestrate cytokinetic events, such as sister nuclei being kept apart, furrow ingression, and abscission. Despite this important role, many aspects of midzone biology remain unknown, including the dynamic organization of midzone microtubules. Investigating midzone microtubule dynamics has been difficult in part because their plus ends are interdigitated and buried in a dense matrix, making them difficult to observe. Result We employed monopolar cytokinesis to reveal that midzone plus ends appear to be nondynamic. We identified the chromokinesin KIF4 as a negative regulator of midzone plus-end dynamics whose activity controls midzone length but not stability. KIF4 is required to terminate midzone elongation in late anaphase. In the absence of KIF4, midzones elongate abnormally, and their overlap regions are unfocused. Electron-dense material and midbodies are both absent from the elongated midzones, and actin filaments from the furrow cortex are not disassembled after ingression. Conclusion KIF4-mediated midzone length regulation appears to occur by terminating midzone elongation at a specific time during cytokinesis, making midzones and mitotic spindles differ in their dynamics and length-regulating mechanisms.
• C. elegans Notch Signaling Regulates Adult Chemosensory Response and Larval Molting Quiescence
  - Curr Bio 21(10):825-834 (2011)
  Background The conserved DOS-motif proteins OSM-7 and OSM-11 function as coligands with canonical DSL (Delta, Serrate, and LAG-2) ligands to activate C. elegans Notch receptors during development. We report here that Notch ligands, coligands, and the receptors LIN-12 and GLP-1 regulate two C. elegans behaviors: chemosensory avoidance of octanol and quiescence during molting lethargus. Results C. elegans lacking osm-7 or osm-11 are defective in their response to octanol. We find that OSM-11 is secreted from hypodermal seam cells into the pseudocoelomic body cavity and acts non-cell autonomously as a diffusible factor. OSM-11 acts with the DSL ligand LAG-2 to activate LIN-12 and GLP-1 Notch receptors in the neurons of adult animals, thereby regulating octanol avoidance response. In adult animals, overexpression of osm-11 and consequent Notch receptor activation induces anachronistic sleep-like quiescence. Perturbation of Notch signaling alters basal activity in adults as well as arousal thresholds and quiescence during molting lethargus. Genetic epistasis studies reveal that Notch signaling regulates quiescence via previously identified circuits and genetic pathways including the egl-4 cGMP-dependent kinase. Conclusions Our findings indicate that the conserved Notch pathway modulates behavior in adult C. elegans in response to environmental stress. Additionally, Notch signaling regulates sleep-like quiescence in C. elegans, suggesting that Notch may regulate sleep in other species.
• Notch Signaling Modulates Sleep Homeostasis and Learning after Sleep Deprivation in Drosophila
  - Curr Bio 21(10):835-840 (2011)
  The role of the transmembrane receptor Notch in the adult brain is poorly understood. Here, we provide evidence that bunched, a negative regulator of Notch, is involved in sleep homeostasis. Genetic evidence indicates that interfering with bunched activity in the mushroom bodies (MBs) abolishes sleep homeostasis. Combining bunched and Delta loss-of-function mutations rescues normal homeostasis, suggesting that Notch signaling may be involved in regulating sensitivity to sleep loss. Preventing the downregulation of Delta by overexpressing a wild-type transgene in MBs reduces sleep homeostasis and, importantly, prevents learning impairments induced by sleep deprivation. Similar resistance to sleep loss is observed with Notchspl-1 gain-of-function mutants. Immunohistochemistry reveals that the Notch receptor is expressed in glia, whereas Delta is localized in neurons. Importantly, the expression in glia of the intracellular domain of Notch, a dominant activated form of th! e receptor, is sufficient to prevent learning deficits after sleep deprivation. Together, these results identify a novel neuron-glia signaling pathway dependent on Notch and regulated by bunched. These data highlight the emerging role of neuron-glia interactions in regulating both sleep and learning impairments associated with sleep loss.
• Blue Light-Dependent Interaction of CRY2 with SPA1 Regulates COP1 activity and Floral Initiation in Arabidopsis
  - Curr Bio 21(10):841-847 (2011)
  Cryptochromes are blue light receptors that mediate light regulation of gene expression in all major evolution lineages, but the molecular mechanism underlying cryptochrome signal transduction remains not fully understood [[1] and [2]]. It has been reported that cryptochromes suppress activity of the multifunctional E3 ubiquitin ligase CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1) to regulate gene expression in response to blue light [[3] and [4]]. But how plant cryptochromes mediate light suppression of COP1 activity remains unclear. We report here that Arabidopsis CRY2 (cryptochrome 2) undergoes blue light-dependent interaction with the COP1-interacting protein SUPPRESSOR OF PHYTOCHROME A 1 (SPA1) [[5] and [6]]. We demonstrate that SPA1 acts genetically downstream from CRY2 to mediate blue light suppression of the COP1-dependent proteolysis of the flowering-time regulator CONSTANS (CO) [[7] and [8]]. We further show that blue light-dependent CRY2-SPA1 interaction stimulates! CRY2-COP1 interaction. These results reveal for the first time a wavelength-specific mechanism by which a cryptochrome photoreceptor mediates light regulation of protein degradation to modulate developmental timing in Arabidopsis.
• Heterotypic Gap Junctions between Two Neurons in the Drosophila Brain Are Critical for Memory
  - Curr Bio 21(10):848-854 (2011)
  Gap junctions play an important role in the regulation of neuronal metabolism and homeostasis by serving as connections that enable small molecules to pass between cells and synchronize activity between cells [[1], [2] and [3]]. Although recent studies have linked gap junctions to memory formation [[4] and [5]], it remains unclear how they contribute to this process [[1] and [5]]. Gap junctions are hexameric hemichannels formed from the connexin and pannexin gene families in chordates and the innexin (inx) gene family in invertebrates [[6] and [7]]. Here we show that two modulatory neurons, the anterior paired lateral (APL) neuron and the dorsal paired medial (DPM) neuron, form heterotypic gap junctions within the mushroom body (MB), a learning and memory center in the Drosophila brain. Using RNA interference-mediated knockdowns of inx7 and inx6 in the APL and DPM neurons, respectively, we found that flies showed normal olfactory associative learning and intact anesthe! sia-resistant memory (ARM) but failed to form anesthesia-sensitive memory (ASM). Our results reveal that the heterotypic gap junctions between the APL and DPM neurons are an essential part of the MB circuitry for memory formation, potentially constituting a recurrent neural network to stabilize ASM.
• A Pair of Inhibitory Neurons Are Required to Sustain Labile Memory in the Drosophila Mushroom Body
  - Curr Bio 21(10):855-861 (2011)
  Labile memory is thought to be held in the brain as persistent neural network activity [[1], [2], [3] and [4]]. However, it is not known how biologically relevant memory circuits are organized and operate. Labile and persistent appetitive memory in Drosophila requires output after training from the α′β′ subset of mushroom body (MB) neurons and from a pair of modulatory dorsal paired medial (DPM) neurons [[5], [6], [7], [8] and [9]]. DPM neurons innervate the entire MB lobe region and appear to be pre- and postsynaptic to the MB [[7] and [8]], consistent with a recurrent network model. Here we identify a role after training for synaptic output from the GABAergic anterior paired lateral (APL) neurons [[10] and [11]]. Blocking synaptic output from APL neurons after training disrupts labile memory but does not affect long-term memory. APL neurons contact DPM neurons most densely in the α′β′ lobes, although their processes are intertwined and contact throughout ! all of the lobes. Furthermore, APL contacts MB neurons in the α′ lobe but makes little direct contact with those in the distal α lobe. We propose that APL neurons provide widespread inhibition to stabilize and maintain synaptic specificity of a labile memory trace in a recurrent DPM and MB α′β′ neuron circuit.
• Cofilin Tunes the Nucleotide State of Actin Filaments and Severs at Bare and Decorated Segment Boundaries
  - Curr Bio 21(10):862-868 (2011)
  Actin-based motility demands the spatial and temporal coordination of numerous regulatory actin-binding proteins (ABPs) [1], many of which bind with affinities that depend on the nucleotide state of actin filament. Cofilin, one of three ABPs that precisely choreograph actin assembly and organization into comet tails that drive motility in vitro [2], binds and stochastically severs aged ADP actin filament segments of de novo growing actin filaments [3]. Deficiencies in methodologies to track in real time the nucleotide state of actin filaments, as well as cofilin severing, limit the molecular understanding of coupling between actin filament chemical and mechanical states and severing. We engineered a fluorescently labeled cofilin that retains actin filament binding and severing activities. Because cofilin binding depends strongly on the actin-bound nucleotide, direct visualization of fluorescent cofilin binding serves as a marker of the actin filament nucleotide state d! uring assembly. Bound cofilin allosterically accelerates Pi release from unoccupied filament subunits, which shortens the filament ATP/ADP-Pi cap length by nearly an order of magnitude. Real-time visualization of filament severing indicates that fragmentation scales with and occurs preferentially at boundaries between bare and cofilin-decorated filament segments, thereby controlling the overall filament length, depending on cofilin binding density.
• Proteasome Function Is Required for Biological Timing throughout the Twenty-Four Hour Cycle
  - Curr Bio 21(10):869-875 (2011)
  Circadian clocks were, until recently, seen as a consequence of rhythmic transcription of clock components, directed by transcriptional/translational feedback loops (TTFLs). Oscillations of protein modification were then discovered in cyanobacteria [[1] and [2]]. Canonical posttranslational signaling processes have known importance for clocks across taxa [[3], [4], [5], [6], [7], [8], [9], [10] and [11]]. More recently, evidence from the unicellular eukaryote Ostreococcus tauri revealed a transcription-independent, rhythmic protein modification [12] shared in anucleate human cells [13]. In this study, the Ostreococcus system reveals a central role for targeted protein degradation in the mechanism of circadian timing. The Ostreococcus clockwork contains a TTFL involving the morning-expressed CCA1 and evening-expressed TOC1 proteins [14]. Cellular CCA1 and TOC1 protein content and degradation rates are analyzed qualitatively and quantitatively using luciferase reporter f! usion proteins. CCA1 protein degradation rates, measured in high time resolution, feature a sharp clock-regulated peak under constant conditions. TOC1 degradation peaks in response to darkness. Targeted protein degradation, unlike transcription and translation, is shown to be essential to sustain TTFL rhythmicity throughout the circadian cycle. Although proteasomal degradation is not necessary for sustained posttranslational oscillations in transcriptionally inactive cells, TTFL and posttranslational oscillators are normally coupled, and proteasome function is crucial to sustain both.
• Improvement and Impairment of Visually Guided Behavior through LTP- and LTD-like Exposure-Based Visual Learning
  - Curr Bio 21(10):876-882 (2011)
  Cellular studies have focused on long-term potentiation (LTP) and long-term depression (LTD) to understand requirements for persistent changes in synaptic connections [[1], [2] and [3]]. Whereas LTP is induced through high-frequency intermittent stimulation, low-frequency stimulation evokes LTD [4]. Because of the ubiquitous efficacy of these protocols, they are considered fundamental mechanisms underlying learning. Here we adapted LTP/LTD-like protocols to visual stimulation to alter human visually guided behavior. In a change-detection task, participants reported luminance changes against distracting orientation changes. Subsequently, they were exposed to passive visual high- or low-frequency stimulation of either the relevant luminance or irrelevant orientation feature. LTP-like high-frequency protocols using luminance improved ability to detect luminance changes, whereas low-frequency LTD-like stimulation impaired performance. In contrast, LTP-like exposure of the ! irrelevant orientation feature impaired performance, whereas LTD-like orientation stimulation improved it. LTP-like effects were present for 10 days, whereas LTD-like effects lasted for a shorter period of time. Our data demonstrate that instead of electrically stimulating synapses, selective behavioral changes are evoked in humans by using equivalently timed visual stimulation, suggesting that both LTD- and LTP-like protocols control human behavior but that the direction of changes is determined by the feature incorporated into the stimulation protocol.
• TRPM Channels Modulate Epileptic-like Convulsions via Systemic Ion Homeostasis
  - Curr Bio 21(10):883-888 (2011)
  Neuronal networks operate over a wide range of activity levels, with both neuronal and nonneuronal cells contributing to the balance of excitation and inhibition. Activity imbalance within neuronal networks underlies many neurological diseases, such as epilepsy [1]. The Caenorhabditis elegans locomotor circuit operates via coordinated activity of cholinergic excitatory and GABAergic inhibitory transmission [2]. We have previously shown that a gain-of-function mutation in a neuronal acetylcholine receptor, acr-2(gf), causes an epileptic-like convulsion behavior [3]. Here we report that the behavioral and physiological effects of acr-2(gf) require the activity of the TRPM channel GTL-2 in nonneuronal tissues. Loss of gtl-2 function does not affect baseline synaptic transmission but can compensate for the excitation-inhibition imbalance caused by acr-2(gf). The compensatory effects of removing gtl-2 are counterbalanced by another TRPM channel, GTL-1, and can be recapitula! ted by acute treatment with divalent cation chelators, including those specific for Zn2+. Together, these data reveal an important role for ion homeostasis in the balance of neuronal network activity and a novel function of nonneuronal TRPM channels in the fine-tuning of this network activity.
• The Requirement for the Dam1 Complex Is Dependent upon the Number of Kinetochore Proteins and Microtubules
  - Curr Bio 21(10):889-896 (2011)
  The Dam1 complex attaches the kinetochore to spindle microtubules and is a processivity factor in vitro [[1] and [2]]. In Saccharomyces cerevisiae, which has point centromeres that attach to a single microtubule, deletion of any Dam1 complex member results in chromosome segregation failures and cell death [[3], [4] and [5]]. In Schizosaccharomyces pombe, which has epigenetically defined regional centromeres that each attach to 3–5 kinetochore microtubules, Dam1 complex homologs are not essential [6]. To determine why the complex is essential in some organisms and not in others, we used Candida albicans, a multimorphic yeast with regional centromeres that attach to a single microtubule [7]. Interestingly, the Dam1 complex was essential in C. albicans, suggesting that the number of microtubules per centromere is critical for its requirement. Importantly, by increasing CENP-A expression levels, more kinetochore proteins and microtubules were recruited to the centromeres! , which remained fully functional. Furthermore, Dam1 complex members became less crucial for growth in cells with extra kinetochore proteins and microtubules. Thus, the requirement for the Dam1 complex is not due to the DNA-specific nature of point centromeres. Rather, the Dam1 complex is less critical when chromosomes have multiple kinetochore complexes and microtubules per centromere, implying that it functions as a processivity factor in vivo as well as in vitro.
• Heme Synthesis by Plastid Ferrochelatase I Regulates Nuclear Gene Expression in Plants
  - Curr Bio 21(10):897-903 (2011)
  Chloroplast signals regulate hundreds of nuclear genes during development and in response to stress, but little is known of the signals or signal transduction mechanisms of plastid-to-nucleus (retrograde) signaling [[1] and [2]]. In Arabidopsis thaliana, genetic studies using norflurazon (NF), an inhibitor of carotenoid biosynthesis, have identified five GUN (genomes uncoupled) genes, implicating the tetrapyrrole pathway as a source of a retrograde signal. Loss of function of any of these GUN genes leads to increased expression of photosynthesis-associated nuclear genes (PhANGs) when chloroplast development has been blocked by NF [[3] and [4]]. Here we present a new Arabidopsis gain-of-function mutant, gun6-1D, with a similar phenotype. The gun6-1D mutant overexpresses the conserved plastid ferrochelatase 1 (FC1, heme synthase). Genetic and biochemical experiments demonstrate that increased flux through the heme branch of the plastid tetrapyrrole biosynthetic pathway i! ncreases PhANG expression. The second conserved plant ferrochelatase, FC2, colocalizes with FC1, but FC2 activity is unable to increase PhANG expression in undeveloped plastids. These data suggest a model in which heme, specifically produced by FC1, may be used as a retrograde signal to coordinate PhANG expression with chloroplast development.
• Cofilin Tunes the Nucleotide State of Actin Filaments and Severs at Bare and Decorated Segment Boundaries
  - Curr Bio 21(10):904 (2011)