Chromosome Dynamics during Mitosis

The primary goal of mitosis is to partition duplicated chromosomes into daughter cells. Eukaryotic chromosomes are equipped with two distinct classes of intrinsic machineries, cohesin and condensins, that ensure their faithful segregation during mitosis. Cohesin holds sister chromatids together immediately after their synthesis during S phase until the establishment of bipolar attachments to the mitotic spindle in metaphase. Condensins, on the other hand, attempt to “resolve” sister chromatids by counteracting cohesin. The products of the balancing acts of cohesin and condensins are metaphase chromosomes, in which two rod-shaped chromatids are connected primarily at the centromere. In anaphase, this connection is released by the action of separase that proteolytically cleaves the remaining population of cohesin. Recent studies uncover how this series of events might be mechanistically coupled with each other and intricately regulated by a number of regulatory factors.In eukaryotic cells, genomic DNA is packaged into chromatin and stored in the cell nucleus, in which essential chromosomal processes, including DNA replication and gene expression, take place (Fig. 1, interphase). At the onset of mitosis, the nuclear envelope breaks down and chromatin is progressively converted into a discrete set of rod-shaped structures known as metaphase chromosomes (Fig. 1, metaphase). In each chromosome, a pair of sister kinetochores assembles at its centromeric region, and their bioriented attachment to the mitotic spindle acts as a prerequisite for equal segregation of sister chromatids. The linkage between sister chromatids is dissolved at the onset of anaphase, allowing them to be pulled apart to opposite poles of the cell (Fig. 1, anaphase). At the end of mitosis, the nuclear envelope reassembles around two sets of segregated chromatids, leading to the production of genetically identical daughter cells (Fig. 1, telophase).Open in a separate windowFigure 1.Overview of chromosome dynamics during mitosis. In addition to the crucial role of kinetochore–spindle interactions, an intricate balance between cohesive and resolving forces acting on sister chromatid arms (top left, inset) underlies the process of chromosome segregation. See the text for major events in chromosome segregation.Although the centromere–kinetochore region plays a crucial role in the segregation process, sister chromatid arms also undergo dynamic structural changes to facilitate their own separation. Conceptually, such structural changes are an outcome of two balancing forces, namely, cohesive and resolving forces (Fig. 1, top left, inset). The cohesive force holds a pair of duplicated arms until proper timing of separation, otherwise daughter cells would receive too many or too few copies of chromosomes. The resolving force, on the other hand, counteracts the cohesive force, reorganizing each chromosome into a pair of rod-shaped chromatids. From this standpoint, the pathway of chromosome segregation is regarded as a dynamic process, in which the initially robust cohesive force is gradually weakened and eventually dominated by the resolving force. Almost two decades ago, genetic and biochemical studies for the behavior of mitotic chromosomes converged productively, culminating in the discovery of cohesin (Guacci et al. 1997; Michaelis et al. 1997; Losada et al. 1998) and condensin (Hirano et al. 1997; Sutani et al. 1999), which are responsible for the cohesive and resolving forces, respectively. The subsequent characterizations of these two protein complexes have not only transformed our molecular understanding of chromosome dynamics during mitosis and meiosis, but also provided far-reaching implications in genome stability, as well as unexpected links to human diseases. In this article, I summarize recent progress in our understanding of mitotic chromosome dynamics with a major focus on the regulatory networks surrounding cohesin and condensin. I also discuss emerging topics and attempt to clarify outstanding questions in the field.     

Schwann Cells: Development and Role in Nerve Repair

Schwann cells develop from the neural crest in a well-defined sequence of events. This involves the formation of the Schwann cell precursor and immature Schwann cells, followed by the generation of the myelin and nonmyelin (Remak) cells of mature nerves. This review describes the signals that control the embryonic phase of this process and the organogenesis of peripheral nerves. We also discuss the phenotypic plasticity retained by mature Schwann cells, and explain why this unusual feature is central to the striking regenerative potential of the peripheral nervous system (PNS).The myelin and nonmyelin (Remak) Schwann cells of adult nerves originate from the neural crest in well-defined developmental steps (Fig. 1). This review focuses on embryonic development (for additional information on myelination, see Salzer 2015). We also discuss how the ability to change between differentiation states, a characteristic attribute of developing cells, is retained by mature Schwann cells, and explain how the ability of Schwann cells to change phenotype in response to injury allows the peripheral nervous system (PNS) to regenerate after damage.Open in a separate windowFigure 1.Main transitions in the Schwann cell precursor (SCP) lineage. The diagram shows both developmental and injury-induced transitions. Black uninterrupted arrows, normal development; red arrows, the Schwann cell injury response; stippled arrows, postrepair reformation of myelin and Remak cells. Embryonic dates (E) refer to mouse development. (Modified from Jessen and Mirsky 2012; reprinted, with permission and with contribution from Y. Poitelon and L. Feltri.)     

Break-Induced DNA Replication

Recombination-dependent DNA replication, often called break-induced replication (BIR), was initially invoked to explain recombination events in bacteriophage but it has recently been recognized as a fundamentally important mechanism to repair double-strand chromosome breaks in eukaryotes. This mechanism appears to be critically important in the restarting of stalled and broken replication forks and in maintaining the integrity of eroded telomeres. Although BIR helps preserve genome integrity during replication, it also promotes genome instability by the production of loss of heterozygosity and the formation of nonreciprocal translocations, as well as in the generation of complex chromosomal rearrangements.The break-copy mode of recombination (as opposed to break-join), was initially proposed by Meselson and Weigle (1961). Break-copy recombination, now more commonly known as recombination-dependent DNA replication or break-induced replication (BIR), is believed to account for restarting replication at broken replication forks and may also play a central role in the maintenance of telomeres in the absence of telomerase. BIR has been studied in various model systems and has been invoked to explain chromosome rearrangements in humans. This review focuses primarily on mechanistic studies in Escherichia coli and its bacteriophages, T4 and λ, in the budding yeasts Saccharomyces cerevisiae and Kluyveromyces lactis and on apparently similar, but less well-documented, mechanisms in mammalian cells.Homology-dependent repair of DNA double-strand breaks (DSBs) occur by three major repair pathways (Pâques and Haber 1999) (Fig. 1). When both ends of the DNA share substantial homology with a donor template (a sister chromatid, a homologous chromosome, or an ectopically located segment), repair occurs almost exclusively by gene conversion (GC). If the DSB is flanked by direct repeats, then a second repair process, single-strand annealing (SSA), can occur as 5′ to 3′ resection of the DSB ends exposes complementary sequences that can anneal to each other and repair the break by the formation of a deletion. However, when only one DSB end shares homology with a donor sequence, repair occurs by BIR. There are two BIR pathways, one dependent on Rad51 recombinase and the other independent of Rad51.Open in a separate windowFigure 1.Three major repair pathways of homology-dependent recombination. Noncrossover (NCO) and crossover (CO) events are indicated. Black triangles represent resolution of Holliday junctions (HJs). Dashed lines represent new DNA synthesis. GC, gene conversion; SSA, single-strand annealing; BIR, break-induced replication.     

Apoptotic and Nonapoptotic Caspase Functions in Animal Development

A developing animal is exposed to both intrinsic and extrinsic stresses. One stress response is caspase activation. Caspase activation not only controls apoptosis but also proliferation, differentiation, cell shape, and cell migration. Caspase activation drives development by executing cell death or nonapoptotic functions in a cell-autonomous manner, and by secreting signaling molecules or generating mechanical forces, in a noncell autonomous manner.Programmed cell death or apoptosis occurs widely during development. During C. elegans development, 131 cells die by caspase CED-3-dependent apoptosis; however, ced-3 mutants do not show significant developmental defects (Ellis and Horvitz 1986). In contrast, studies on caspase mutants in mouse and Drosophila have revealed caspases’ roles in development. During development, cells are exposed to extrinsic and intrinsic stresses, and caspases are activated as one of multiple stress responses that ensure developmental robustness (Fig. 1). Caspases actively regulate animal development through both apoptosis and nonapoptotic functions that involve cell–cell communication in developing cell communities (Miura 2011). This chapter focuses on the in vivo roles of caspases in development and regeneration.Open in a separate windowFigure 1.Caspase activation during development. An embryo undergoes intrinsic and extrinsic stress, which activates caspases to execute both apoptotic and nonapoptotic functions, including cell differentiation and dendrite pruning. Apoptotic cells affect the shape and behavior of their neighboring cells. Caspase-activated cells are shown in dark gray.     

Auxin and Monocot Development

Monocots are known to respond differently to auxinic herbicides; hence, certain herbicides kill broadleaf (i.e., dicot) weeds while leaving lawns (i.e., monocot grasses) intact. In addition, the characters that distinguish monocots from dicots involve structures whose development is controlled by auxin. However, the molecular mechanisms controlling auxin biosynthesis, homeostasis, transport, and signal transduction appear, so far, to be conserved between monocots and dicots, although there are differences in gene copy number and expression leading to diversification in function. This article provides an update on the conservation and diversification of the roles of genes controlling auxin biosynthesis, transport, and signal transduction in root, shoot, and reproductive development in rice and maize.Auxinic herbicides have been used for decades to control dicot weeds in domestic lawns (Fig. 1A), commercial golf courses, and acres of corn, wheat, and barley, yet it is not understand how auxinic herbicides selectively kill dicots and spare monocots (Grossmann 2000; Kelley and Reichers 2007). Monocots, in particular grasses, must perceive or respond differently to exogenous synthetic auxin than dicots. It has been proposed that this selectivity is because of either limited translocation or rapid degradation of exogenous auxin (Gauvrit and Gaillardon 1991; Monaco et al. 2002), altered vascular anatomy (Monaco et al. 2002), or altered perception of auxin in monocots (Kelley and Reichers 2007). To explain these differences, there is a need to further understand the molecular basis of auxin metabolism, transport, and signaling in monocots.Open in a separate windowFigure 1.Differences between monocots and dicots. (A) A dicot weed in a lawn of grasses. Note the difference in morphology of the leaves. (B) Germinating dicot (bean) seedling. Dicots have two cotyledons (cot). Reticulate venation is apparent in the leaves. The stem below the cotyledons is called the hypocotyl (hyp). (C) Germinating monocot (maize) seedling. Monocots have a single cotyledon called the coleoptile (col) in grasses. Parallel venation is apparent in the leaves. The stem below the coleoptile is called the mesocotyl (mes).Auxin, as we have seen in previous articles, plays a major role in vegetative, reproductive, and root development in the model dicot, Arabidopsis. However, monocots have a very different anatomy from dicots (Raven et al. 2005). Many of the characters that distinguish monocots and dicots involve structures whose development is controlled by auxin: (1) As the name implies, monocots have single cotyledons, whereas dicots have two cotyledons (Fig. 1B,C). Auxin transport during embryogenesis may play a role in this difference as cotyledon number defects are often seen in auxin transport mutants (reviewed in Chandler 2008). (2) The vasculature in leaves of dicots is reticulate, whereas the vasculature in monocots is parallel (Fig. 1). Auxin functions in vascular development because many mutants defective in auxin transport, biosynthesis, or signaling have vasculature defects (Scarpella and Meijer 2004). (3) Dicots often produce a primary tap root that produces lateral roots, whereas, in monocots, especially grasses, shoot-borne adventitious roots are the most prominent component of the root system leading to the characteristic fibrous root system (Fig. 2). Auxin induces lateral-root formation in dicots and adventitious root formation in grasses (Hochholdinger and Zimmermann 2008).Open in a separate windowFigure 2.The root system in monocots. (A) Maize seedling showing the primary root (1yR), which has many lateral roots (LR). The seminal roots (SR) are a type of adventitious root produced during embryonic development. Crown roots (CR) are produced from stem tissue. (B) The base of a maize plant showing prop roots (PR), which are adventitious roots produced from basal nodes of the stem later in development.It is not yet clear if auxin controls the differences in morphology seen in dicots versus monocots. However, both conservation and diversification of mechanisms of auxin biosynthesis, homeostasis, transport, and signal transduction have been discovered so far. This article highlights the similarities and the differences in the role of auxin in monocots compared with dicots. First, the genes in each of the pathways are introduced (Part I, Table I) and then the function of these genes in development is discussed with examples from the monocot grasses, maize, and rice (Part II).     

The Spatiotemporal Organization of ErbB Receptors: Insights from Microscopy

Signal transduction is regulated by protein–protein interactions. In the case of the ErbB family of receptor tyrosine kinases (RTKs), the precise nature of these interactions remains a topic of debate. In this review, we describe state-of-the-art imaging techniques that are providing new details into receptor dynamics, clustering, and interactions. We present the general principles of these techniques, their limitations, and the unique observations they provide about ErbB spatiotemporal organization.Signal transduction is associated with dramatic spatial and temporal changes in membrane protein distribution. Although the biochemical events downstream of membrane receptor activation are often well characterized, the initiating events within the plasma membrane remain unclear. Many cell surface receptors have been shown to redistribute into clusters in response to ligand binding (Metzger 1992). Therefore, correlating membrane receptor activation with dynamics and aggregation state is essential to understanding cell signaling.The role of receptor aggregation is of particular interest in the case of receptor tyrosine kinases (RTKs). It is generally accepted that ligand binding to the extracellular domain of RTKs induces dimerization, whether ligand- or receptor-mediated (Lemmon and Schlessinger 2010). However, there is evidence that some RTKs exist as oligomers in the absence of ligand, whereas others require higher-order oligomerization for activation (Lemmon and Schlessinger 2010). Understanding the fundamental interactions that regulate RTK signaling still remains an important focus in the field.Over the past decade, imaging technologies and biological tools have developed to a point such that questions about protein dynamics, clustering, and interactions can now be addressed in living cells (Fig. 1). These techniques reveal information about protein behavior on a spatial and temporal scale that is not provided by traditional biochemical assays. In this review, we will discuss the application of these advanced imaging technologies to the study of the ErbB family of RTKs.Open in a separate windowFigure 1.Summary of imaging techniques for quantifying receptor clustering, dynamics, and interactions.     

Polarity in Stem Cell Division: Asymmetric Stem Cell Division in Tissue Homeostasis

Many adult stem cells divide asymmetrically to balance self-renewal and differentiation, thereby maintaining tissue homeostasis. Asymmetric stem cell divisions depend on asymmetric cell architecture (i.e., cell polarity) within the cell and/or the cellular environment. In particular, as residents of the tissues they sustain, stem cells are inevitably placed in the context of the tissue architecture. Indeed, many stem cells are polarized within their microenvironment, or the stem cell niche, and their asymmetric division relies on their relationship with the microenvironment. Here, we review asymmetric stem cell divisions in the context of the stem cell niche with a focus on Drosophila germ line stem cells, where the nature of niche-dependent asymmetric stem cell division is well characterized.Asymmetric cell division allows stem cells to self-renew and produce another cell that undergoes differentiation, thus providing a simple method for tissue homeostasis. Stem cell self-renewal refers to the daughter(s) of stem cell division maintaining all stem cell characteristics, including proliferation capacity, maintenance of the undifferentiated state, and the capability to produce daughter cells that undergo differentiation. A failure to maintain the correct stem cell number has been speculated to lead to tumorigenesis/tissue hyperplasia via stem cell hyperproliferation or tissue degeneration/aging via a reduction in stem cell number or activity (Morrison and Kimble 2006; Rando 2006). This necessity changes during development. The stem cell pool requires expansion earlier in development, whereas maintenance is needed later to sustain tissue homeostasis.There are two major mechanisms to sustain a fixed number of adult stem cells: stem cell niche and asymmetric stem cell division, which are not mutually exclusive. Stem cell niche is a microenvironment in which stem cells reside, and provides essential signals required for stem cell identity (Fig. 1A). Physical limitation of niche “space” can therefore define stem cell number within a tissue. Within such a niche, many stem cells divide asymmetrically, giving rise to one stem cell and one differentiating cell, by placing one daughter inside and another outside of the niche, respectively (Fig. 1A). Nevertheless, some stem cells divide asymmetrically, apparently without the niche. For example, in Drosophila neuroblasts, cell-intrinsic fate determinants are polarized within a dividing cell, and subsequent partitioning of such fate determinants into daughter cells in an asymmetric manner results in asymmetric stem cell division (Fig. 1B) (see Fig. 3A and Prehoda 2009).Open in a separate windowFigure 1.Mechanisms of asymmetric stem cell division. (A) Asymmetric stem cell division by extrinsic fate determinants (i.e., the stem cell niche). The two daughters of stem cell division will be placed in distinct cellular environments either inside or outside the stem cell niche, leading to asymmetric fate choice. (B) Asymmetric stem cell division by intrinsic fate determinants. Fate determinants are polarized in the dividing stem cells, which are subsequently partitioned into two daughter cells unequally, thus making the division asymmetrical. Self-renewing (red line) and/or differentiation promoting (green line) factors may be involved.In this review, we focus primarily on asymmetric stem cell divisions in the Drosophila germ line as the most intensively studied examples of niche-dependent asymmetric stem cell division. We also discuss some examples of stem cell division outside Drosophila, where stem cells are known to divide asymmetrically or in a niche-dependent manner.     

DNA Replication Origins

The onset of genomic DNA synthesis requires precise interactions of specialized initiator proteins with DNA at sites where the replication machinery can be loaded. These sites, defined as replication origins, are found at a few unique locations in all of the prokaryotic chromosomes examined so far. However, replication origins are dispersed among tens of thousands of loci in metazoan chromosomes, thereby raising questions regarding the role of specific nucleotide sequences and chromatin environment in origin selection and the mechanisms used by initiators to recognize replication origins. Close examination of bacterial and archaeal replication origins reveals an array of DNA sequence motifs that position individual initiator protein molecules and promote initiator oligomerization on origin DNA. Conversely, the need for specific recognition sequences in eukaryotic replication origins is relaxed. In fact, the primary rule for origin selection appears to be flexibility, a feature that is modulated either by structural elements or by epigenetic mechanisms at least partly linked to the organization of the genome for gene expression.Timely duplication of the genome is an essential step in the reproduction of any cell, and it is not surprising that chromosomal DNA synthesis is tightly regulated by mechanisms that determine precisely where and when new replication forks are assembled. The first model for a DNA synthesis regulatory circuit was described about 50 years ago (Jacob et al. 1963), based on the idea that an early, key step in building new replication forks was the binding of a chromosomally encoded initiator protein to specialized DNA regions, termed replication origins (Fig. 1). The number of replication origins in a genome is, for the most part, dependent on chromosome size. Bacterial and archaeal genomes, which usually consist of a small circular chromosome, frequently have a single replication origin (Barry and Bell 2006; Gao and Zhang 2007). In contrast, eukaryotic genomes contain significantly more origins, ranging from 400 in yeast to 30,000–50,000 in humans (Cvetic and Walter 2005; Méchali 2010), because timely duplication of their larger linear chromosomes requires establishment of replication forks at multiple locations. The interaction of origin DNA and initiator proteins (Fig. 1) ultimately results in the assembly of prereplicative complexes (pre-RCs), whose role is to load and activate the DNA helicases necessary to unwind DNA before replication (Remus and Diffley 2009; Kawakami and Katayama 2010). Following helicase-catalyzed DNA unwinding, replisomal proteins become associated with the single-stranded DNA, and new replication forks proceed bidirectionally along the genome until every region is duplicated (for review, see O’Donnell 2006; Masai et al. 2010).Open in a separate windowFigure 1.Revised versions of the replicon model for all domains of life. For cells of each domain type, trans-acting initiators recognize replication origins to assemble prereplicative complexes required to unwind the DNA and load DNA helicase. Eukaryotic initiators are preassembled into hexameric origin recognition complexes (ORCs) before interacting with DNA. In prokaryotes, single initiators (archaeal Orc1/Cdc6 or bacterial DnaA) bind to recognition sites and assemble into complexes on DNA. In all cases, the DNA helicases (MCMs or DnaB) are recruited to the origin and loaded onto single DNA strands. In bacteria, DNA-bending proteins, such as Fis or IHF, may modulate the assembly of pre-RC by bending the origin DNA. Two activities of DnaA are described in the figure. The larger version binds to recognition sites, and the smaller version represents DnaA required to assist DnaC in loading DnaB helicase on single-stranded DNA.Initiator proteins from all forms of life share structural similarities, including membership in the AAA⁺ family of proteins (ATPases associated with various cellular activities) (Duderstadt and Berger 2008; Wigley 2009) that are activated by ATP binding and inactivated by ATP hydrolysis (Duderstadt and Berger 2008; Duncker et al. 2009; Kawakami and Katayama 2010). Despite these similarities, initiators assemble into prereplicative complexes in two fundamentally different ways (Fig. 2). In prokaryotes, initiator monomers interact with the origin at multiple repeated DNA sequence motifs, and the arrangement of these motifs (see below) can direct assembly of oligomers that mediate strand separation (Erzberger et al. 2006; Rozgaja et al. 2011). In eukaryotes, a hexameric origin recognition complex (ORC) binds to replication origins and then recruit additional factors (as Cdc6 and Cdt1) that will themselves recruit the hexameric MCM2-7 DNA helicase to form a prereplicative complex (for review, see Diffley 2011). This process occurs during mitosis and along G₁ and is called “DNA replication licensing,” a crucial regulation of eukaryotic DNA replication (for review, see Blow and Gillespie 2008). Importantly, this complex is still inactive, and only a subset of these preassembled origins will be activated in S phase. This process is, therefore, fundamentally different from initiation of replication in bacteria. Moreover, because sequence specificity appears more relaxed in large eukaryotic genomes, prokaryotic mechanisms that regulate initiator–DNA site occupation must be replaced by alternative mechanisms, such as structural elements or the use of epigenetic factors.Open in a separate windowFigure 2.Functional elements in some well-studied prokaryotic replication origins. (A) Bacterial oriCs. The DNA elements described in the text are (arrows) DnaA recognition boxes or (boxes) DNA unwinding elements (DUEs). When recognition site affinities are known, colored arrows designate high- (K_d > 100 nm) and low- (K_d < 100 nm) affinity sites. (B) Archaeal oriCs. Arrows and boxes designate DNA elements as in A, but the initiator protein is Orc1/Cdc6 rather than DnaA. (Thick arrows) Long origin recognition boxes (ORBs); (thin arrows) shorter versions (miniORBs). Both ORBs and miniORBs are identified in Pyrococcus. DUEs are not yet well defined for Helicobacter or Sulfolobus genera and are not labeled in this figure.Here, we describe replication origins on prokaryotic and eukaryotic genomes below, with a particular focus on the attributes responsible for orderly initiator interactions and origin selection specificity, as well as on the shift from origin sequence-dependent regulation to epigenetic regulation. You are also referred to other related articles in this collection and several recent reviews covering the topics of DNA replication initiation in more detail (Méchali 2010; Beattie and Bell 2011; Blow et al. 2011; Bryant and Aves 2011; Ding and MacAlpine 2011; Dorn and Cook 2011; Kaguni 2011; Leonard and Grimwade 2011; Sequeira-Mendes and Gomez 2012).     

The Role of Functional Prion-Like Proteins in the Persistence of Memory

Prions are a self-templating amyloidogenic state of normal cellular proteins, such as prion protein (PrP). They have been identified as the pathogenic agents, contributing to a number of diseases of the nervous system. However, the discovery that the neuronal RNA-binding protein, cytoplasmic polyadenylation element-binding protein (CPEB), has a prion-like state that is involved in the stabilization of memory raised the possibility that prion-like proteins can serve normal physiological functions in the nervous system. Here, we review recent experimental evidence of prion-like properties of neuronal CPEB in various organisms and propose a model of how the prion-like state may stabilize memory.Prions are proteinaceous infectious agents that were discovered in the 1980s by Stanley Prusiner while studying Creutzfeldt–Jakob disease (Prusiner 1982). Prusiner and colleagues showed them to be an amyloidogenic, self-perpetuating, forms of a normal cellular protein, termed prion protein or PrP. Prp in its self-perpetuating state kills cells. Prusiner and colleagues found that PrPs exist in at least two conformations: monomeric and aggregated (Fig. 1). The transition among these forms occurs spontaneously and only the aggregated conformation is pathogenic. Soon, PrPs were found to contribute to other neurodegenerative disorders in people, including kuru, transmissible spongiform encephalopathies, as well as bovine spongiform encephalopathy in cows (Prusiner 1994; Aguzzi and Weissmann 1998).Open in a separate windowFigure 1.Pathogenic prions exist in two states (soluble and aggregated and self-perpetuating). The conversion from the soluble to the aggregated form is spontaneous and the aggregated, self-perpetuating form is often toxic and kills the cell.There is now a growing consensus that similar prion-like, self-templating mechanisms underlie a variety of neurodegenerative disorders, including amyotrophic lateral sclerosis, Alzheimer’s disease, Parkinson’s disease, and Huntington’s disease (Polymenidou and Cleveland 2012).Not all prions, however, appear to be disease causing. Fungal prions, for instance, are nontoxic, and some may even be beneficial to the cells that harbor them (Wickner 1994; Shorter and Lindquist 2005; Crow and Li 2011). In 2003, Si and Kandel serendipitously discovered a prion-like protein in multicellular eukaryotes—the nervous system of the marine snail Aplysia—whose aggregated and self-perpetuating form contributes to the maintenance of long-term changes in synaptic efficacy. This functional prion-like protein differs from pathogenic prions in two important ways: (1) The conversion to the prion-like state is regulated by a physiological signal, and (2) the aggregated form has an identified physiological function (Fig. 2). Recent identification of new functional prion-like proteins in various organisms, including human, supports the idea that nonpathogenic prions may perform a wide range of biologically meaningful roles (Coustou et al. 1997; Eaglestone et al. 1999; True and Lindquist 2000; Ishimaru et al. 2003; True et al. 2004; Hou et al. 2011; Jarosz et al. 2014).Open in a separate windowFigure 2.“Functional” prion: memory. “Functional” prions differ from conventional prions in two ways. First, the conversion is triggered by a physiological signal, and second, the aggregated, self-perpetrating forms have a physiological function. 5-HT, Serotonin; DA, dopamine.In this review, we focus on functional prion-like proteins in the brain and specifically on the prion-like properties of the cytoplasmic polyadenylation element-binding protein (CPEB), and examine how the prion-like state can control protein synthesis at the synapse and, thereby, synaptic plasticity and long-lasting memory. We anticipate the studies of CPEB would also provide some generalizable concepts as to how prion-based protein switches in multicellular eukaryotes may work.     

The Signaling Mechanisms Underlying Cell Polarity and Chemotaxis

Chemotaxis—the directed movement of cells in a gradient of chemoattractant—is essential for neutrophils to crawl to sites of inflammation and infection and for Dictyostelium discoideum (D. discoideum) to aggregate during morphogenesis. Chemoattractant-induced activation of spatially localized cellular signals causes cells to polarize and move toward the highest concentration of the chemoattractant. Extensive studies have been devoted to achieving a better understanding of the mechanism(s) used by a neutrophil to choose its direction of polarity and to crawl effectively in response to chemoattractant gradients. Recent technological advances are beginning to reveal many fascinating details of the intracellular signaling components that spatially direct the cytoskeleton of neutrophils and D. discoideum and the complementary mechanisms that make the cell''s front distinct from its back.Chemotaxis—the directed movement of cells in a gradient of chemoattractant—allows leukocytes to seek out sites of inflammation and infection, amoebas of Dictyostelium discoideum (D. discoideum) to aggregate, neurons to send projections to specific regions of the brain to find their synaptic partners, yeast cells to mate, and fibroblasts to move into the wound space (Fig. 1). In each case, chemoattractant-induced activation of spatially localized cellular signals causes cells to polarize and move toward the highest concentration of the chemoattractant. During chemotaxis, filamentous actin (F-actin) is polymerized asymmetrically at the upgradient edge of the cell (leading edge), providing the necessary force to thrust projections of the plasma membrane in the proper direction (see Mullins 2009). Neutrophilic leukocytes (neutrophils), for instance, can polarize and move up very shallow gradients, with a chemoattractant concentration ∼2% higher at the front than the back (Fig. 2) (Devreotes and Zigmond 1988). To restrict actin polymerization to the leading edge in such a shallow gradient, neutrophils must create a much steeper internal gradient of regulatory signals. In addition, distinctive actin–myosin contractile complexes are also formed at the sides and back of the cells (Fig. 2). The ability to create such distinctive segregation of actin assemblies enables neutrophils to move nearly 50 times more quickly than fibroblasts. The polarization response is self-organizing, which occurs even when the attractant concentration is uniform and apparently stimulating all portions of the plasma membrane at the same intensity; in the absence of a gradient, the direction of polarity is random, but all cells can be induced to polarize (Fig. 2). Thus, neutrophil polarization to chemoattractant stimulation represents a striking example of symmetry breaking from an unpolarized state to a polarized one.Open in a separate windowFigure 1.Examples of chemotaxis. (A) A human neutrophil chasing a Staphylococcus aureus microorganism on a blood film among red blood cells, notable for their dark color and principally spherical shape (imaged by David Rogers, courtesy of Thomas P. Stossel). Bar, 10 µm. Chemotaxis is also necessary for (B) D. discoideum to form multicellular aggregates during development (courtesy of M.J. Grimson and R.L. Blanton, Texas Tech University), and (C) for axons to find their way in the developing nervous system. Photo provided by Kathryn Tosney, University of Miami.Open in a separate windowFigure 2.(A–D) Polarization of a neutrophil in response to gradient of chemoattractant. Nomarski images of unpolarized neutrophil responding to a micropipette containing the chemoattractant fMLP (white circle) at (A) 5 s, (B) 30 s, (C) 81 s, and (D) 129 s of stimulation. Bar = 5 µm. (Figure is taken from Weiner et al. 1999, with permission.) Human neutrophils stimulated with fMLP show highly polarized morphology and asymmetric cytoskeletal assemblies. (E–G) Human neutrophils were stimulated by a uniform concentration of fMLP (100 nM) and fixed 2 min after stimulations. Fixed cells were stained for F-actin with rhodamine-phalloidin (E, red) and an antibody raised against activated myosin II (phosphorylated specifically at Ser19, p[19]-MLC) (F, green). These fluorescent images are merged with Nomarski image in (G). Bars, 10 µm.To enter an infected tissue, neutrophils require chemoattractants produced by host cells and microorganisms to migrate to the sites and infection and inflammation. Neutrophil chemotaxis also contributes to many inflammatory and autoimmune diseases, including rheumatoid arthritis, ischemia-reperfusion syndrome, acute respiratory distress, and systemic inflammatory response syndromes. Although the critical physiological functions of neutrophils have made their chemoattractants and chemoattractant receptors targets of intense investigation, understanding of the neutrophil polarity and directional migration has until recently lagged behind that of other cells. Over the past decade, experimentation with knockout mice and human neutrophil cell lines has begun to shed light on the complex intracellular signals responsible for neutrophil polarity. In this article, I summarize recent advances in the study of chemotactic signals in neutrophils, with some of the discussion also devoted to a related model—chemotaxis of D. discoideum. These soil amoebas grow as single cells, but on starvation chemotax into multicellular aggregates in response to secreted chemoattractants such as adenosine 3′,5′-monophosphate (cAMP).     

Cosmic Carbon Chemistry: From the Interstellar Medium to the Early Earth

Astronomical observations have shown that carbonaceous compounds in the gas and solid state, refractory and icy are ubiquitous in our and distant galaxies. Interstellar molecular clouds and circumstellar envelopes are factories of complex molecular synthesis. A surprisingly large number of molecules that are used in contemporary biochemistry on Earth are found in the interstellar medium, planetary atmospheres and surfaces, comets, asteroids and meteorites, and interplanetary dust particles. In this article we review the current knowledge of abundant organic material in different space environments and investigate the connection between presolar and solar system material, based on observations of interstellar dust and gas, cometary volatiles, simulation experiments, and the analysis of extraterrestrial matter. Current challenges in astrochemistry are discussed and future research directions are proposed.Carbon is a key element in the evolution of prebiotic material (Henning and Salama 1998), and becomes biologically interesting in compounds with nitrogen, oxygen and hydrogen. Our understanding of the evolution of organic molecules—including such compounds—and their voyage from molecular clouds to the early solar system and Earth provides important constraints on the emergence of life on Earth and possibly elsewhere (Ehrenfreund and Charnley 2000). Figure 1 shows the cycle of organic molecules in the universe. Gas and solid-state chemical reactions form a variety of organic molecules in circumstellar and interstellar environments. During the formation of the solar system, this interstellar organic material was chemically processed and later integrated in the presolar nebula from which planets and small solar system bodies formed. The remnant planetesimals in the form of comets and asteroids impacted the young planets in the early history of the solar system (Gomes et al. 2005). The large quantities of extraterrestrial material delivered to young planetary surfaces during the heavy bombardment phase may have played a key role in life''s origin (Chyba and Sagan 1992, Ehrenfreund et al. 2002). How elements are formed, how complex carbonaceous molecules in space are, what their abundance is and on what timescales they form are crucial questions within cosmochemistry.Open in a separate windowFigure 1.Carbon pathways between interstellar and circumstellar regions and the forming solar system.