Plasma Membrane
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Quantitative Modeling of Membrane Transport and Anisogamy by Small Groups Within a Large-Enrollment Organismal Biology Course †
- Authors: Eric S. Haag, Gili Marbach-Ad*
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Citation: Haag E, Marbach-Ad G. 2016. Quantitative modeling of membrane transport and anisogamy by small groups within a large-enrollment organismal biology course † . 17(3):485-486 doi:10.1128/jmbe.v17i3.1192
- DOI 10.1128/jmbe.v17i3.1192
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Abstract:
Quantitative modeling is not a standard part of undergraduate biology education, yet is routine in the physical sciences. Because of the obvious biophysical aspects, classes in anatomy and physiology offer an opportunity to introduce modeling approaches to the introductory curriculum. Here, we describe two in-class exercises for small groups working within a large-enrollment introductory course in organismal biology. Both build and derive biological insights from quantitative models, implemented using spreadsheets. One exercise models the evolution of anisogamy (i.e., small sperm and large eggs) from an initial state of isogamy. Groups of four students work on Excel spreadsheets (from one to four laptops per group). The other exercise uses an online simulator to generate data related to membrane transport of a solute, and a cloud-based spreadsheet to analyze them. We provide tips for implementing these exercises gleaned from two years of experience.
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Secretion from Myeloid Cells: Secretory Lysosomes
- Author: Gillian M. Griffiths
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Citation: Griffiths G. 2016. Secretion from myeloid cells: secretory lysosomes. 4(4): doi:10.1128/microbiolspec.MCHD-0030-2016
- DOI 10.1128/microbiolspec.MCHD-0030-2016
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Abstract:
Many cells of the myeloid lineage use an unusual secretory organelle to deliver their effector mechanisms. In these cells, the lysosomal compartment is often modified not only to fulfill the degradative functions of a lysosome but also as a mechanism for secreting additional proteins that are found in the lysosomes of each specialized cell type. These extra proteins vary from one cell type to another according to the specialized function of the cell. For example, mast cells package histamine; cytotoxic T cells express perforin; azurophilic granules in neutrophils express antimicrobial peptides, and platelets von Willebrand factor. Upon release, these very different proteins can trigger inflammation, cell lysis, microbial death, and clotting, respectively, and hence deliver the very different effector mechanisms of these different myeloid cells.
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New Insights on Eisosomes
- Publication Date : October 2015
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Fungal eisosomes are shallow, trough-shaped invaginations of the plasma membrane, of unknown function, that are ubiquitous in fungi. Fungal eisosome assembly requires two conserved proteins carrying “BAR” domains, triple-coiled-coil motifs associated with generation of membrane curvature. Now Ursula Goodenough of Washington University, St. Louis, et al. have identified eisosomes in a subset of red and green microalgae and in cysts of a ciliate. “Microalgal eisosome assembly is correlated with the presence and nature of cell wells,” she says. Though sequenced microalgae lack fungal BAR proteins, she has identified two lineage-specific BAR-encoding gene families that are candidate eisosome organizers. “The presence of eisosomes in algae, fungi, and ciliates indicates that these membrane differentiations were present in ancient eukaryotic common ancestors,” she says. “Experiments probing function in microalgae may yield functional insights. Some fungicides are known to bind to ergosterol, which is enriched in fungal eisosomes.” And this, she says, “is the first report of a stable structural patterning of membrane
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Detection and Clinical Significance of Variability among Candida Isolates
- Author: Lois L. Hoyer
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Source: Candida and Candidiasis, Second Edition , pp 91-99
Publication Date :
January 2012
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Abstract:
Strain variability is a central topic in the discussion of Candida biology. Most of the Candida isolates that are studied are derived from clinical specimens. Therefore, a discussion of variability among Candida isolates within a particular species starts with decisions that are made in the diagnostic microbiology laboratory. Studies of Candida genetic variability may also be conducted from the perspective of assessing genomic rearrangements. The nature of genomic rearrangements in the diploid Candida albicans was understood more clearly from construction of a physical map of its eight pairs of chromosomes. DNA fingerprinting probe sequences useful in phylogenetic and epidemiological studies are derived from the major repeat sequence (MRS). The MRS is also present in the closely related Candida dubliniensis. Events such as mutation and mitotic recombination also can contribute to variability among C. albicans strains. One study examined whether the local wildlife population was responsible for maintaining a reservoir of C. albicans isolates specific to a defined geographic area in the midwestern United States. The work was expanded to include collection of C. albicans isolates from domestic animals. Results showed that there is a significant difference in the clade distribution of isolates from humans and wildlife, demonstrating population isolation between the groups. The work demonstrates the impressive display of genetic variability that C. albicans can develop when challenged with exposure to antifungal drugs. A given isolate of C. albicans can undergo an impressive range of genetic changes at the level of point mutation to alterations in whole chromosomes.
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Cell Cycle and Growth Control in Candida Species
- Authors: Cheryl A. Gale, Judith Berman
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Source: Candida and Candidiasis, Second Edition , pp 101-124
Publication Date :
January 2012
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Abstract:
This chapter describes the major Candida albicans morphologies and the current understanding of the cell biological and cell cycle features that distinguish them. It highlights recent insights into how cell cycle regulators influence the formation of hypha-specific cellular features in particular. Since morphogenesis and cell cycle regulation have been studied most extensively in C. albicans, the chapter primarily focuses on work in C. albicans. The important distinction between yeast and pseudohyphae is that pseudohyphae spend more time in G2 phase of the cell cycle than yeast cells , and they continue to elongate during this time. There has long been a controversy as to how pseudohyphae are related to true hyphae. Initial models suggested that yeast cells, pseudohyphae, and true hyphae reside along a continuum. Later, based on differences in cell cycle dynamics and subcellular structures, it was proposed that pseudohyphae and hyphae represent two distinct morphological states, with pseudohyphae being more like yeast form growth with respect to cell cycle progression and cell biological markers. Recent work has shed light on cell biological features associated with cell cycle progression in chlamydospores and is discussed in theis chapter. In the C. albicans genome sequence, there are three G1 cyclins (Ccn1, Cln3, and Hgc1) and two G2 or B-type/mitotic cyclins (Clb2 and Clb4) that are predicted to associate with Cdc28.
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Initial Interactions of Chlamydiae with the Host Cell
- Author: Ted Hackstadt
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Source: Intracellular Pathogens I: Chlamydiales , pp 126-148
Publication Date :
January 2012
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Chlamydiae are taken up by host cells so efficiently that the process has been termed "parasite-specified endocytosis" by Byrne and Moulder. This chapter focuses upon the active interactions of chlamydiae with the host cell and covers the stage of development up to the division of reticulate bodies (RBs). Surprisingly, different mutant cell lines displayed unique patterns of susceptibility or resistance to different C. trachomatis serovars or to C. pneumoniae. Despite the difficulties of comparing studies between different chlamydial species or serovars, conditions, and cell types, there is sufficient evidence to suggest that individual chlamydiae are capable of utilizing different mechanisms for entry. Translocation of Tarp and its tyrosine phosphorylation appear to be one of the first means of communication with the host cell to actively subvert host processes for parasite purposes. Identification of the kinases mediating Tarp phosphorylation is an initial step in mapping the signal transduction networks initiated by C. trachomatis to establish residence within an intracellular niche. Endocytosed elementary bodies (EBs) are rapidly transported to the peri-Golgi region of the host cell and become fusogenic with Golgi-derived vesicles. The Inc proteins do not share structural similarities to eukaryotic proteins, except for SNARE-like motifs, and homology searches do not provide substantial clues about their function. An improved understanding of the complex interactions of the important pathogens with the host cell should provide great potential for improved chemotherapeu-tic and immunoprophylactic interventions.
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Biosynthesis and Genetics of the Cryptococcus Capsule
- Authors: Guilhem Janbon, Tamara L. Doering
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Source: Cryptococcus , pp 27-41
Publication Date :
January 2011
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Until the late 1940s, little was known about cryptococcal capsule composition or structure. Classically, the capsule is described as composed of glucuronoxylomannan (GXM), galactoxylomannan (GalXM), and mannoproteins, based on the original fractionation of shed polysaccharide material. Cell wall polymers are involved in capsule association with the cell, although they are not considered part of the capsule itself. Polysaccharide synthesis starts with the generation of precursor molecules, the most common being the activated sugars discovered by Leloir. A number of proteins involved in the synthesis of these precursors have been identified and studied in Cryptococcus neoformans. This work has been facilitated by the fact that many of these enzymes are highly conserved in terms of sequence, as would be expected from the participation of activated sugar precursors in multiple synthetic pathways across biology. In support of a lumenal location for capsule synthesis, a conditional mutant generated in both serotypes A and D that is defective in vesicle targeting to the plasma membrane accumulates post-Golgi vesicles containing GXM. The conclusion from this study is that GXM is made within the classical secretory pathway, consistent with the requirement for nucleotide sugar transporters to achieve normal capsule synthesis.
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Innate Immunity to Viruses
- Author: Akiko Iwasaki
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Source: The Immune Response to Infection , pp 185-196
Publication Date :
January 2011
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Viruses are the most abundant infectious agents on earth and the most primitive form of life, predating us by billions of years. Due to the fact that viruses utilize host cell machinery to replicate, the host has the daunting task of distinguishing virus infection signatures from self-molecules. This chapter provides an overview of the mechanisms used to sense virus infection (innate recognition), the cytokine system that evolved to rapidly induce antiviral states in neighboring cells (type I interferons), and the methods used to contain and destroy viruses (effector functions). Analysis of animals deficient in the RIG-I-like receptors (RLRs) revealed important and distinct roles for the sensing molecules in innate immunity. DNA viruses such as adenovirus stimulate the NLRP3-ASC-caspase-1 inflammasomes in vivo. The existence of a TLR-independent cytoplasmic DNA sensing molecule leading to type I IFN production was suggested from studies utilizing DNA viruses and bacteria. The mechanism of RNAi involves two steps. First, viral dsRNA is recognized by Dicer-like endonuclease family, which processes it into siRNA. Second, the siRNA are incorporated into RNA-induced silencing complex (RISC), which guide the RNase enzyme AGO to complementary sequences (viral RNA) for cleavage and degradation of viral RNA. Tetherin associates with lipid rafts and inhibits retrovirus particle release in the absence of Vpu. Vpu utilizes the beta-TrCP E3 ubiquitin ligase complex to induce endosomal trafficking events that remove tetherin from the cell surface, rendering it incapable of restricting the release of enveloped viruses.
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Antibiotics and the Apicoplast
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Source: Magic Bullets to Conquer Malaria , pp 182-203
Publication Date :
January 2011
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The discovery of the apicoplast heightened and broadened the therapeutic role of antibiotics, some of which were already useful in malaria chemotherapy and prophylaxis. Although the apicoplast was definitively identified in 1997, it had actually been seen in the early 1960s by those studying malaria parasites under electron microscopes. Researchers hypothesized that the plastid-like DNA had arisen when the ancestor of the malaria parasites took up an algal cell and then incorporated it into its own body. The discovery of streptomycin was the catalyst for the pharmaceutical industry to engage in a race to find new antibiotics. Tetracyclines were produced by the golden-colored Streptomyces aureofaciens and were called aureomycin. Doxycycline had an advantage over some other teracyclines in that it is rapidly removed by the liver and hence does not produce kidney toxicity. Researchers concluded that the antibiotics with distinct mechanisms of action in prokaryotes inhibited the survival of Plasmodium falciparum in vitro by virtue of their action on the mitochondrion. This chapter also discusses the effects of some antibiotics on the apicoplast ribosome. Treatment of malaria patients with antimalarial drug combinations that include azithromycin has shown promise in clinical trials because of the effect of these drugs on the apicoplast.
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Yeast Cytology, 1950 to 1990
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Source: Yeast Research , pp 60-75
Publication Date :
January 2011
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This chapter summarizes selected and somewhat disparate advances made in yeast cytology after about 1950. Northcote and Horne disintegrated their baker’s yeast mechanically, and after centrifuging, mounted the cell wall fraction in polyvinyl formal films. The yeast walls proved to be stratified: after acid hydrolysis, chromatography showed that the outer layer was mainly mannan-protein; the walls contained 29% glucan and 31% mannan, previously reported for yeast cell walls as 13% protein and 8.5% lipid . It was in 1956 that Necas described the formation of some protoplasts or spheroplasts amongst spontaneously autolyzing S. cerevisiae. In 1989, Hartwell emphasized the relevance of this work on the cell cycle for the prevention of mammalian cancers. Chromosome behaviour in meiosis is well characterized from cytological and genetic descriptions but little is known of the underlying molecular mechanisms, largely because no one experimental system has been developed to support an integrated application of modern cytological, genetic, and molecular biological methods. A thorough account of the amount of work published on yeast cytology in the last 50 or so years would occupy several volumes; in addition, no account has been given here of advances made since 1990, which involve such innovations as the green fluorescent protein reporter system, confocal microscopy, and flow cytometric analysis.
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Yeast Taxonomy, 1900 to 2000
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Source: Yeast Research , pp 254-274
Publication Date :
January 2011
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As well as isolating new species, taxonomists have, almost uniquely undertaken the comparative study and description of many different kinds of yeast, and this is probably one of their most valuable contributions to yeast biology. This chapter considers some aspects of the history of all these kinds of activity. Ascospore-forming yeasts, such as Saccharomyces species, were isolated notably from industrial fermentations, whereas many non-ascospore-producing yeasts were found in clinical practice-for example, Candida albicans and Malassezia furfur, often the putative causes of mycotic diseases. In 1924, Endomyces fibuliger was transferred to Saccharomycopsis. However, the genus Saccharomycopsis generally lapsed into desuetude until Kreger-van Rij restored it in all its glory in 1984. Writing from the Faculté de Médecine of Paris in 1932, Maurice Langeron and Rodolfo Talice published a paper on classifying those fungi which characteristically formed both filaments and yeast-like cells. This paper was largely a report of a microscopical study of the different categories of cell produced by each kind of yeast: blastoconidia, chlamydospores, the mode of budding, and the greatly varied appearance of filamentous growths. The chapter describes the inception of some genera which were thought to be asexual, namely, Brettanomyces, Sporobolomyces, Bullera, Rhodotorula, Kloeckera, Trigonopsis, and Schizoblastosporion. The sensible naming of yeasts is vital for all who work with them, in research, in commerce, and in medicine.
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Hyphal Structure
- Authors: Robert W. Roberson, Maritza Abril, Meredith Blackwell, Peter Letcher, David J. McLaughlin, Rosa R. Mouri ÑO-PÉrez, Meritxell Riquelme, Maho Uchida
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Source: Cellular and Molecular Biology of Filamentous Fungi , pp 8-24
Publication Date :
January 2010
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Abstract:
The ability to understand cytoplasmic structure can provide powerful insights into the biology of cells and organisms. This chapter has briefly reviewed the diversity of hyphal structures and presented examples of how bioimaging has contributed to a broader understanding of hyphal biology and phylogenetic relationships between fungal taxa. At the heart of polarized growth is the secretory pathway in which vesicles are targeted to sites of growth and subsequently fuse with the plasma membrane. In mature hyphae of the septate fungi, these events have given rise to the Spitzenkörper, a complex and dynamic structure that clearly influences hyphal growth and morphogenesis. The presence or absence of certain morphological characters (e.g., septa and Woronin bodies) already has been useful in defining higher taxa, especially since evolutionary polarity often can be established using stable phylogenetic trees based on DNA sequences. Ever-enlarging molecular databases, especially those of whole genomes, are allowing us to look for the genetic basis of many structural features, such as the presence or absence of Woronin body matrix proteins. This capability will allow us to understand the basis of these features not only in an evolutionary sense but also in a functional one. Collaboration among different types of fungal biologists including systematists is essential to understanding structure and how it applies to the study of the Fungi.
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Peroxisomes in Filamentous Fungi
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Source: Cellular and Molecular Biology of Filamentous Fungi , pp 191-206
Publication Date :
January 2010
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This chapter presents an overview summarizing the current knowledge of the function of peroxisomes in filamentous fungi. Peroxisome formation among diverse eukaryotic organisms shares a common basic biogenetic process mediated by a number of conserved proteins known as peroxins. At the start of photosynthesis, peroxisome metabolism plays an important role in photorespiration, and peroxisomes have thus been described as "leaf peroxisomes". The chapter mentions the main metabolic peroxisomal functions of fungi, with emphasis on those that are particular to, and have been characterized more fully in, filamentous fungi. Plants, yeasts, and filamentous fungi display a wide spectrum of peroxisomal activities, mainly due to the existence of peroxisome-specific function. In addition to the fruiting-body constitution, peroxisomes also participate in differentiation processes taking place in the fertile portion (centrum) of these structures. P. anserina strains lacking the peroxisome-targeting signal (PTS) receptors peroxisomal matrix 5 (PEX5) and PEX7 exhibit abnormal formation of asci, resulting in ascospores with uneven numbers of nuclei or spores with no nuclei. The second centrum developmental event in which peroxisomes are involved in P. anserina is the transition from the prekaryogamy mitotic phase to the karyogamy and meiotic phase. In spite of the progress made in understanding peroxisome function in fungi, a detailed picture of how peroxisomes affect several other metabolic and developmental processes remains elusive. Further innovative approaches are required to fully understand the function of this organelle in fungi.
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The Cytoskeleton in Filamentous Fungi
- Authors: Xin Xiang, Berl Oakley
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Source: Cellular and Molecular Biology of Filamentous Fungi , pp 209-223
Publication Date :
January 2010
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This chapter focuses on the microtubule and actin cytoskeletons of filamentous fungi, including the motor proteins that are integral to cytoskeletal function. Relevant results from yeasts are discussed to provide background and context. The emphasis is on more recent data from live-cell imaging as well as genetic and molecular genetic studies. It has been shown that both the microtubule and the actin cytoskeletons play roles in polarized growth of hyphae, and how these cytoskeletal elements function to support hyphal growth and organelle distribution in elongated hyphae is a topic of great interest. Dynein in filamentous fungi also participates in organizing the microtubule network by regulating microtubule dynamics and by providing force for transporting microtubules. In filamentous fungi, the actin cytoskeleton and its myosin motors are important for the delivery of cell membrane and cell wall components to the growing hyphal tip and to the septum. Myosins are a diverse superfamily of actin motor proteins that play various cellular roles. In filamentous fungi such as A. nidulans and N. crassa, four families of myosins have been found, including myosin-I, myosin-II, myosin-V, and the fungus-specific chitin synthases with myosin motor domains. Hyphal growth in filamentous fungi needs both microtubule and actin cytoskeletons, and thus, it would be important to understand how these two systems interact to coordinate vesicle transport towards the hyphal tip.
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Regulation of Gene Expression by Ambient pH
- Authors: Joan Tilburn, Herbert N. Arst, Jr., Miguel A. Peñalva
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Source: Cellular and Molecular Biology of Filamentous Fungi , pp 480-487
Publication Date :
January 2010
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Ambient pH regulation of gene expression is a transcriptional regulatory system that enables the presence of gene products appropriate to the pH of the environment and prevents those that are inappropriate to the environmental pH. Much of the progress in elucidating the mechanism of ambient pH gene regulation has been made using Aspergillus nidulans, and there are a number of reasons why this organism is particularly favorable for the study of pH regulation. This chapter discusses some of these reasons and presents a brief description of the current model of fungal pH regulation. Mutations resulting in gene expression appropriate to acidic environments confer intense staining for acid phosphatase activity even with growth at alkaline pH but prevent staining for alkaline phosphatase activity. The product of the signaling proteolysis, PacC53, is thought to be accessible to the processing protease due to an open conformation assumed in the absence of interacting region C, and the processing proteolysis occurs in a pH-independent manner, removing a further ~250 C-terminal residues to form PacC27. In Aspergillus nidulans and in unicellular yeasts, the pH signaling pathway involves six dedicated proteins. Significant advances in one's current understanding of pH signaling have come from research using budding yeast as an experimental model. Compelling evidence obtained in studies of Saccharomyces cerevisiae and A. nidulans strongly supports the existence of a second pH signaling complex associated to the endosomal sorting complex required for transport (ESCRT) complexes on endosomal membranes.
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Introduction to Second Messengers: Lessons from Cyclic AMP
- Author: Alan J. Wolfe
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Source: The Second Messenger Cyclic Di-GMP , pp 3-7
Publication Date :
January 2010
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At the turn of the 21st century, researchers linked the second messenger bis-(3',5') cyclic diguanylic acid (also known as cyclic di-GMP [c-di-GMP]) to proteins that contained either the GGDEF domain, the EAL domain, or both and showed that these proteins were ubiquitous. Researchers had reported that epinephrine was produced by the adrenal gland and that this epinephrine traveled from the adrenal gland to the liver cells that store glycogen; however, the mechanism by which epinephrine elicited this effect remained unknown. Today, the mammalian cyclic AMP (cAMP) signal transduction network is known to include a dizzying array of G-protein-coupled receptors, a plethora of G-protein subunits, 10 adenylyl cyclases (9 membrane bound and 1 cytosolic), 11 PDE families, and multiple cAMP effectors, including PKA, PKC, diverse guanine exchange factors, and a variety of cyclic nucleotide-gated ion channels. The parallels between the mammalian cAMP network and the bacterial systems centered on c-di-GMP are striking. One obvious reason to compartmentalize proteins within microdomains is to bring related signaling components into close proximity. The mammalian plasma membrane is heterogeneous. This heterogeneity is produced, in part, by the concentration to specific locations of large amounts of cholesterol and sphingolipids. Two major hypotheses have been proposed to explain the ability of PDEs to shape cAMP gradients: the barrier hypothesis and the sink hypothesis.
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Bioreactor Automation
- Authors: David Hopkins, Melissa St. Anad, Jack Prior
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Source: Manual of Industrial Microbiology and Biotechnology, Third Edition , pp 719-730
Publication Date :
January 2010
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The goal of bioreactor control is to manage the cells’ environment and metabolism to maximize productivity and product quality. This chapter introduces basic automation terminology and concepts as well as the rationale for implementing various levels of automation. Before designing or purchasing an automation system, the full range of process operations and requirements must be understood. Major operations associated with a given batch typically consist of equipment preparation and assembly, sterilization, inoculation, cell growth and production, and cleaning. Bioreactor measurements can be categorized by the physical location of the sensor relative to the process they are measuring. Typical in-line measurements of reactor conditions include pH, dissolved oxygen (DO), dissolved carbon dioxide, pressure, temperature, weight/level, foam level, and agitation speed. Historically, various measurements at-line or off-line are used to monitor cell growth and metabolism of the cell culture. Most bioreactor automation uses a single-input, single-output controller, which consists of a sensor input, a controller, and a control device/actuator. The controller calculates an output to send to the control device(s) based on the current and historical values obtained from the controlled variable (CV) relative to the desired set point (SP). Cultures with very high O2 demand may also require the pressure of the bioreactor to be increased, which causes the concentration of all dissolved gases to increase.
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Signaling for Phagocytosis
- Author: Joel A. Swanson
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Source: Phagocyte-Pathogen Interactions , pp 195-208
Publication Date :
January 2009
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Phagocytes ingest a variety of particles, cells, and microbes by use of actin-rich, contractile extensions of plasma membrane that close into intracellular membranous organelles called phagosomes. Signaling for phagocytosis begins when cell surface receptors engage particle-bound molecular ligands. The distinct component activities localize to subregions of forming phagosomes and may be coordinated by membrane-associated proteins and phospholipids that diffuse in the plane of the membrane bilayer. Different receptors use distinct mechanisms for regulating the actin cytoskeleton and the movements of membrane, producing a variety of morphologies for forming phagosomes. Some elements of phagocytic signaling are both constructive and interpretive. This chapter emphasizes the constructive signals. The cytosolic signaling proteins bind to the receptors or associated proteins, and a complex aggregate assembles around the receptor by diffusion and trapping. Secondary signals include the enzymatic modification of phospholipids and other membrane-associated proteins that in turn organize the cytoplasmic movements of phagocytosis. The underlying mechanism of phagocytosis appears to depend less on localized extension of the plasma membrane and more on controlled invagination of the cell surface by actin-myosin contractile activities. The morphogenesis of phagosomes suggests that actinmyosin-like contractility is essential to phagocytosis. The chapter reviews the signals for various different receptor-mediated phagocytic processes. The patterns of GTPase activities and the corresponding patterns of movement for actin, myosin, and membranes may be coordinated by patterns of membrane lipids in the inner leaflet of the inner membrane of the phagocytic cup that appear and disappear through the course of phagocytosis.
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Actin-Based Motility in Professional Phagocytes †
- Author: Frederick S. Southwick
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Source: Phagocyte-Pathogen Interactions , pp 235-248
Publication Date :
January 2009
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Phagocytes are among the most dynamic cells in the body. Within seconds of exposure to a chemotactic peptide, they are able to convert from a rounded symmetric shape to a polarized morphology. Actin polymerization not only drives the motility of all mammalian nonmuscle cells, but also provides the propulsive force for the movement of intracellular bacteria including Listeria, Shigella, and Rickettsia, as well as the poxvirus vaccinia. Actin is the most abundant protein in the cytoplasm of phagocytes, representing 10 to 20% of the total cytoplasmic protein. The cytoplasm of phagocytes contains high concentrations of unpolymerized actin that exceed by several orders of magnitude the macroscopic critical concentration of purified actin. The factor primarily responsible for preventing actin monomer assembly into filaments is the 5-kDa polypeptide known as thymosin β4 (Tβ4). This small protein binds to a single actin monomer to form a 1:1, or binary, complex. High rates of actin assembly are required to produce the rapid changes in shape observed during phagocyte amoeboid movement. Adherence receptors contain high concentrations of actin filaments and are linked to the actin cytoskeleton by ezrin, radixin, and moesin (the ERM proteins). Proteins that bind to the barbed ends of actin filaments have a profound effect on filament growth. The barbed end has a high affinity for actin monomers and in combination with profilin can readily compete with Tβ4 for sequestered ATP-actin monomers.
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Autophagy: a Fundamental Cytoplasmic Sanitation Process Operational in All Cell Types Including Macrophages
- Author: Vojo Deretic
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Source: Phagocyte-Pathogen Interactions , pp 419-425
Publication Date :
January 2009
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Autophagic degradation is a cell-autonomous mechanism for direct elimination of intracellular microbes, but also plays a broader role in innate and adaptive immunity and in general cytoplasmic homeostasis. The process of autophagy plays many functions, including feeding the cells under starvation conditions or upon withdrawal of growth factors. Autophagy plays both housekeeping and immune functions in macrophages and dendritic cells. The phagophore is enlarged by the addition of a new membrane that is of undefined origin but is suspected to come from the endoplasmic reticulum or a combination of sources including Golgi and endosomes. Most cells undergo baseline autophagy, removing protein aggregates and spuriously damaged mitochondria or other organelles, or adjusting the cellular biomass. The classical physiological inducers of autophagy are amino acid starvation or absence of growth factors. Recent studies examining effects on the redistribution of a sole integral membrane Atg protein, Atg9, have implicated ULK1 as the putative mammalian Atg1 ortholog. A classical physiological inducer of autophagy is amino acid starvation, in particular, withdrawal of leucine. In mammalian cells, the role of autophagy has been linked to immunological signals. In mice and humans it is under the control of cytokines and agonists regulating innate and adaptive immunity, such as interferon gamma (IFN-γ), tumor necrosis factor alpha (TNF-α), and interleukin 13 (IL-13), and CD40L-CD40.