Showing papers in "International Review of Cytology-a Survey of Cell Biology in 2000"

Cytoarchitecture and physical properties of cytoplasm: volume, viscosity, diffusion, intracellular surface area.

Abstract: Classical biochemistry is founded on several assumptions valid in dilute aqueous solutions that are often extended without question to the interior milieu of intact cells. In the first section of this chapter, we present these assumptions and briefly examine the ways in which the cell interior may depart from the conditions of an ideal solution. In the second section, we summarize experimental evidence regarding the physical properties of the cell cytoplasm and their effect on the diffusion and binding of macromolecules and vesicles. While many details remain to be worked out, it is clear that the aqueous phase of the cytoplasm is crowded rather than dilute, and that the diffusion and partitioning of macromolecules and vesicles in cytoplasm is highly restricted by steric hindrance as well as by unexpected binding interactions. Furthermore, the enzymes of several metabolic pathways are now known to be organized into structural and functional units with specific localizations in the solid phase, and as much as half the cellular protein content may also be in the solid phase.

Cell biology of aluminum toxicity and tolerance in higher plants.

Abstract: Aluminum is the major element in the soil and exists as a stable complex with oxygen and silicate in neutral and weakly acidic soil. When the soil pH is lower than 4.5-5.0, Al is solubilized in the soil water and absorbed by plant roots. Absorbed Al inhibits root elongation severely, and the elongation of roots exposed to Al3+ as low as mumol level is inhibited within an hour(s). Thus much research has been conducted to understand the mechanism of Al toxicity and tolerance. Al is located specifically at the root apex. Al-sensitive plants absorb more Al than do Al-tolerant plants, and thus the exclusion mechanism of Al is the major idea for Al tolerance. The understanding of Al stress in plants is important for stable food production in future. Al is a complicated ion in its chemical form and biological function. In this chapter, mechanisms of Al toxicity and tolerance proposed during the past few decades as well as future topics are described from physiological and molecular points of view.

Calcium signaling during abiotic stress in plants.

Abstract: Plants experience a wide array of environmental stimuli, not all of which are favorable, and, unlike animals, are unable to move away from stressful environments. They therefore require a mechanism with which to recognize and respond to abiotic stresses of many different types. Frequently this mechanism involves intracellular calcium. Stress-induced changes in the cytosolic concentration of Ca2+ ([Ca2+]cyt) occur as a result of influx of Ca2+ from outside the cell, or release of Ca2+ from intracellular stores. These alterations in [Ca2+]cyt constitute a signal that is transduced via calmodulin, calcium-dependent protein kinases, and other Ca(2+)-controlled proteins to effect a wide array of downstream responses involved in the protection of the plant and adjustment to the new environmental conditions. Ca2+ signaling has been implicated in plant responses to a number of abiotic stresses including low temperature, osmotic stress, heat, oxidative stress, anoxia, and mechanical perturbation, which are reviewed in this article.

Eph receptors and ephrins: regulators of guidance and assembly.

Abstract: Recent advances have started to elucidate the developmental functions and biochemistry of Eph receptor tyrosine kinases and their membrane-bound ligands, ephrins. Interactions between these molecules are promiscuous, but they largely fall into two groups: EphA receptors bind to GPI-anchored ephrin-A ligands, while EphB receptors bind to ephrin-B proteins that have a transmembrane and cytoplasmic domain. Remarkably, ephrin-B proteins transduce signals, such that bidirectional signaling can occur upon interaction with Eph receptor. In many tissues, specific Eph receptors and ephrins have complementary domains, whereas other family members may overlap in their expression. An important role of Eph receptors and ephrins is to mediate cell-contact-dependent repulsion. Complementary and overlapping gradients of expression underlie establishment of a topographic map of neuronal projections in the retinotectal system. Eph receptors and ephrins also act at boundaries to channel neuronal growth cones along specific pathways, restrict the migration of neural crest cells, and via bidirectional signaling prevent intermingling between hindbrain segments. Intriguingly, Eph receptors and ephrins can also trigger an adhesive response of endothelial cells and are required for the remodeling of blood vessels. Biochemical studies suggest that the extent of multimerization of Eph receptors modulates the cellular response and that the actin cytoskeleton is one major target of the intracellular pathways activated by Eph receptors. Eph receptors and ephrins have thus emerged as key regulators of the repulsion and adhesion of cells that underlie the establishment, maintainence, and remodeling of patterns of cellular organization.

The cell and developmental biology of tendons and ligaments.

Abstract: We have sought to create, for the first time in a single comprehensive review, a modern synthesis of opinion on the cell, developmental, and molecular biology of tendons, ligaments, and their associated structures (tendon sheaths, vinculi, and retinacula). Particular attention has been paid to highlighting new data on the early development of tendons, the signaling molecules involved in their patterning, and the diversity of specialized regions (entheses, wrap-around regions, and myotendinous junctions) that characterize fully formed tendons and ligaments. We have emphasized the complexities of adult tendon and ligament cell shape and related these to their early development. The importance of gap junctions in allowing cell communication throughout an extensive extracellular matrix (ECM) has also been highlighted, particularly in relation to understanding how tendon and ligament cells respond to changes in mechanical load. Finally, we have considered the influence of growth factors and related molecules on cell proliferation and ECM synthesis.

Role of natural benzoxazinones in the survival strategy of plants

Abstract: Benzoxazinoid acetal glucosides are a unique class of natural products abundant in Gramineae, including the major agricultural crops maize, wheat, and rye. These secondary metabolites are also found in several dicotyledonous species. Benzoxazinoids serve as important factors of host plant resistance against microbial diseases and insects and as allelochemicals and endogenous ligands. Interdisciplinary investigations by biologists, biochemists, and chemists are stimulated by the intention to make agricultural use of the benzoxazinones as natural pesticides. These natural products are not only constituents of a plant defense system but also part of an active allelochemical system used in the competition with other plants. This review covers biological and chemical aspects of benzoxazinone research over the last decade with special emphasis on recent advances in the elucidation of the biosynthetic pathway.

Cellulose microfibrils in plants: biosynthesis, deposition, and integration into the cell wall.

Abstract: Cellulose occurs in all higher plants and some algae, fungi, bacteria, and animals. It forms microfibrils containing the crystalline allomorphs, cellulose I alpha and I beta. Cellulose molecules are 500-15,000 glucose units long. What controls molecular size is unknown. Microfibrils are elongated by particle rosettes in the plasma membrane (cellulose synthase complexes). The precursor, UDP-glucose, may be generated from sucrose at the site of synthesis. The biosynthetic mechanism may involve lipid-linked intermediates. Cellulose synthase has been purified from bacteria, but not from plants. In plants, disrupted cellulose synthase may form callose. Cellulose synthase genes have been isolated from bacteria and plants. Cellulose-deficient mutants have been characterised. The deduced amino acid sequence suggests possible catalytic mechanisms. It is not known whether synthesis occurs at the reducing or nonreducing end. Endoglucanase may play a role in synthesis. Nascent cellulose molecules associate by Van der Waals and hydrogen bonds to form microfibrils. Cortical microtubules control microfibril orientation, thus determining the direction of cell growth. Self-assembly mechanisms may operate. Microfibril integration into the wall occurs by interactions with matrix polymers during microfibril formation.

Renewal of photoreceptor outer segments and their phagocytosis by the retinal pigment epithelium.

Abstract: The discovery of disc protein renewal in rod outer segments, in 1960s, was followed by the observation that old discs were ingested by the retinal pigment epithelium This process occurs in both rods and cones and is crucial for their survival Photoreceptors completely degenerate in the Royal College of Surgeons mutant rat, whose pigment epithelium cannot ingest old discs The complete renewal process includes the following sequential steps involving both photoreceptor and pigment epithelium activity: new disc assembly and old disc shedding by photoreceptor cells; recognition and binding to pigment epithelium membranes; then ingestion, digestion, and segregation of residual bodies in pigment epithelium cytoplasm Regulating factors are involved at each step While disc assembly is mostly genetically controlled, disc shedding and the subsequent pigment epithelium phagocytosis appear regulated by environmental factors (light and temperature) Disc shedding is rhythmically controlled by an eye intrinsic circadian oscillator using endogenous dopamine and melatonin as light and dark signal, respectively Of special interest is the regulation of phagocytosis by multiple receptors, including specific phagocytosis receptors and receptors for neuroactive substances released from the neuroretina The candidates for phagocytosis receptors are presented, but it is acknowledged that they are not completely known The main neuromodulators are adenosine, dopamine, glutamate, serotonin, and melatonin Although the transduction mechanisms are not fully understood, attention was brought to cyclic AMP, phosphoinositides, and calcium The chapter points to the multiplicity of regulating factors and the complexity of their intermingling modes of action Promising areas for future research still exist in this field

Structural and Functional Evolution of the Natriuretic Peptide System in Vertebrates

Abstract: The natriuretic peptide (NP) system consists of three types of hormones [atrial NP (ANP), brain or B-type NP (BNP), and C-type NP (CNP)] and three types of receptors [NP receptor (R)-A, NPR-B, and NPR-C]. ANP and BNP are circulating hormones secreted from the heart, whereas CNP is basically a neuropeptide. NPR-A and NPR-B are membrane-bound guanylyl cyclases, whereas NPR-C is assumed to function as a clearance-type receptor. ANP, BNP, and CNP occur commonly in all tetrapods, but ventricular NP replaces BNP in teleost fish. In elasmobranchs, only CNP is found, even in the heart, suggesting that CNP is an ancestral form. A new guanylyl cyclase-uncoupled receptor named NPR-D has been identified in the eel in addition to NPR-A, -B, and -C. The NP system plays pivotal roles in cardiovascular and body fluid homeostasis. ANP is secreted in response to an increase in blood volume and acts on various organs to decrease both water and Na+, resulting in restoration of blood volume. In the eel, however, ANP is secreted in response to an increase in plasma osmolality and decreases Na+ specifically, thereby promoting seawater adaptation. Therefore, it seems that the family of NPs were originally Na(+)-extruding hormones in fishes; however, they evolved to be volume-depleting hormones promoting the excretion of both Na+ and water in tetrapods in which both are always regulated in the same direction. Vertebrates expanded their habitats from fresh water to the sea or to land during evolution. The structure and function of osmoregulatory hormones have also undergone evolution during this ecological evolution. Thus, a comparative approach to the study of the NP family affords new insights into the essential function of this osmoregulatory hormone.

Gonadal development in mammals at the cellular and molecular levels.

Abstract: In mammals, although sex is determined chromosomally, gonads in both sexes begin development as similar structures. Until recently it was widely held that female development constituted a "default" pathway of development, which would occur in the absence of a testis-determining gene. This master gene on the Y chromosome, SRY in the human and Sry in the mouse, is thought to act in a cell-autonomous fashion to determine that cells in the gonadal somatic population develop as pre-Sertoli cells. Triggering of somatic cell differentiation along the Sertoli cell pathway is therefore a key event; it was thought that further steps in gonadal differentiation would follow in a developmental cascade. In the absence of Sertoli cells, the lack of anti-Mullerian hormone would allow development of the female Mullerian duct and absence of Leydig cells would prevent maintenance of the Wolffian duct. Recent findings that female signals not only maintain the Mullerian duct and repress the Wolffian duct but also suppress the development of Leydig cells and maintain meiotic germ cells, together with the finding that an X-linked gene is required for ovarian development and must be silenced in the male, have shown that the female default pathway model is an oversimplification. Morphological steps in gonadal differentiation can be correlated with emerging evidence of molecular mechanisms; growth factors, cell adhesion, and signaling molecules interact together, often acting within short time windows via reciprocal control relationships.

Morphogenesis of the eggshell in Drosophila.

Abstract: The Drosophila eggshell is a specialized extracellular matrix that forms between the oocyte and overlaying somatic follicle cells during the latter stages of oogenesis. Largely proteinaceous, the eggshell is a highly organized multilayered structure with regional specializations designed to perform a variety of functions. Production of a functional eggshell features: (1) the differentiation of subsets of follicle cells in response to ovarian signals, (2) directed migrations of the follicle cells within the developing egg chamber, (3) expression of eggshell structural genes by the follicle cells in a defined temporal and spatial order, (4) postdepositional modifications of the eggshell proteins including several temporally regulated proteolytic cleavage events, and (5) regulated trafficking of several eggshell proteins in the assembling structure. By exploiting the genetic advantages of Drosophila and using evolution as a guide, the eggshell provides an excellent experimental system to study, in vivo, molecular mechanisms used to regulate protein-protein interactions throughout the assembly of a complex extracellular architecture in a developing organism.

Growth factors and epithelial-stromal interactions in prostate cancer development.

Abstract: Epithelial-stromal interactions are important not only in growth, development, and functional cytodifferentiation of the prostate but also in derangements of prostate gland such as BPH and prostate carcinoma. This chapter explores the roles of epithelium and stroma during this delicate process and highlights the role and mutual influence of each on the other. It also examines the importance of ECM in mediating the effects of androgens and drawn attention to estrogen and genetic factors in the process. During this process of epithelial-stromal interaction, growth factors play a central role in mediating the interactions. This chapter focuses on the role of several growth factors including epidermal growth factor, fibroblast growth factor, transforming growth factor alpha, transforming growth factor beta, insulin-like growth factor-1, vascular endothelial growth factor, nerve growth factor, platelet-derived growth factor, and hepatocyte growth factor. This chapter emphasizes the importance of epithelial-stromal interactions in tumorigenesis and highlights the switch of paracrine to autocrine mode during the process of carcinogenesis.

Membrane trafficking and processing in Paramecium.

Abstract: Cellular membranes are made in a cell's biosynthetic pathway and are composed of similar biochemical constituents. Nevertheless, they become differentiated as membrane components are sorted into different membrane-limited compartments. We summarize the morphological and immunological similarities and differences seen in the membranes of the various interacting compartments in the single-celled organism, Paramecium. Besides the biosynthetic pathway, membranes of the regulated secretory pathway, endocytic pathway, and phagocytic pathway are highlighted. Paramecium is a multipolarized cell in the sense that several different pools of membrane-limited compartments are targeted for exocytosis at very specific sites at the cell surface. Thus, the method used by this cell to sort and package its membrane subunits into different compartments, the processes used to transport these compartments to specific locations at the plasma membrane and to other intracellular fusion sites, the processes of membrane retrieval, and the processes of membrane docking and fusion are reviewed. Paramecium has provided an excellent model for studying the complexities of membrane trafficking in one cell using both morphological and immunocytochemical techniques. This cell also promises to be a useful model for studying aspects of the molecular biology of membrane sorting, retrieval, transport, and fusion.

Glial cells as targets and producers of neurotrophins.

Abstract: Glial cells fulfill important tasks within the neural network of the central and peripheral nervous systems. The synthesis and secretion of various polypeptidic factors (cytokines) and a number of receptors, with which glial cells are equipped, allow them to communicate with their environment. Evidence has accumulated during recent years that neurotrophins play an important role not only for neurons but also for glial cells. This brief update of some morphological, immunocytochemical, and biochemical characteristics of glial cell lineages conveys our present knowledge about glial cells as targets and producers of neurotrophins under normal and pathological conditions. The chapter discusses the presence of neurotrophin receptors on glial cells, glial cells as producers of neurotrophins, signaling pathways downstream Trk and p75NTR, and the significance of neurotrophins and their receptors for glial cells during development, in cell death and survival, and in neurological disorders.

Somite formation and patterning.

Abstract: As a consequence of their segmented arrangement and the diversity of their tissue derivatives, somites are key elements in the establishment of the metameric body plan in vertebrates. This article aims to largely review what is known about somite development, from the initial stages of somite formation through the process of somite regionalization along the three major body axes. The role of both cell intrinsic mechanisms and environmental cues are evaluated. The periodic and bilaterally synchronous nature of somite formation is proposed to rely on the existence of a developmental clock. Molecular mechanisms underlying these events are reported. The importance of an antero-posterior somitic polarity with respect to somite formation on one hand and body segmentation on the other hand is discussed. Finally, the mechanisms leading to the regionalization of somites along the dorso-ventral and medio-lateral axes are reviewed. This somitic compartmentalization is believed to underlie the segregation of dermis, skeleton, and dorsal and appendicular musculature.

Cellular responses to vasectomy

Abstract: A number of cell populations in the reproductive tract show a response to vasectomy. Some cell types show similar responses in man and all laboratory species, whereas others show marked species variations. This chapter describes these effects in a broadly chronological order and, in a general way, considers changes close to the site of vasectomy first and the longer term effects on the testis itself later. Following vasectomy, epididymal distension and sperm granuloma formation result from raised intraluminal pressure. The sperm granuloma is a dynamic structure and a site of much spermatozoal phagocytosis by its macrophage population. In many species, spermatozoa in the obstructed ducts are destroyed by intraluminal macrophages, and degradation products, rather than whole sperm, are absorbed by the epididymal epithelium. Humoral immunity against spermatozoal antigens following vasectomy is well established and there is evidence of modest T-lymphocyte activity. The role of lymphocytes in the reproductive tract epithelium and interstitium following vasectomy is poorly defined. In laboratory animals, there is evidence that pressure-mediated damage to the seminiferous epithelium can follow sperm granuloma formation and obstruction in the epididymal head. However, the contribution of lymphocytes and antisperm antibodies to testicular damage after vasectomy is far from clear. A number of studies have suggested that testicular changes may follow vasectomy in man but their validity and mechanism of occurrence require further study.

Compositions and Phase Diagrams for Aqueous Systems Based on Proteins and Polysaccharides

Abstract: Limited thermodynamic compatibility of proteins with other proteins and proteins with polysaccharides is a fundamental phenomenon that has been demonstrated in more than 200 biopolymer pairs. These systems can undergo a liquid-liquid phase separation resulting in the different macromolecular components primarily concentrated in the different phases. This occurs under conditions (pH values and ionic strengths) inhibiting attraction between nonidentical biopolymers, i.e., the formation of interbiopolymer complexes. Generally, phase separation takes place when the total concentration of the macromolecular components exceeds a certain critical value. The excluded volume of the macromolecules determines both their thermodynamic activity and phase separation threshold. Phase diagrams of biopolymer mixtures and physicochemical features of biphasic systems are considered here. Attention is centered on the limited compatibility of the main classes of proteins and various polysaccharides and on the effects of variables such as pH, ionic strength, temperature and shear forces on the phase state, equilibrium and structure of these two-phase liquid systems. The general nature of the phenomenon of thermodynamic incompatibility of biopolymers accounts for its importance in structure formation in cytoplasm.

Involvement of gap junctional communication in myogenesis.

Abstract: Cell-to-cell communication plays important roles in development and in tissue morphogenesis. Gap junctional intercellular communication (GJIC) has been implicated in embryonic development of various tissues and provides a pathway to exchange ions, secondary messengers, and metabolites through the intercellular gap junction channels. Although GJIC is absent in adult skeletal muscles, the formation of skeletal muscles involves a sequence of complex events including cell-cell interaction processes where myogenic cells closely adhere to each other. Much experimental evidence has shown that myogenic precursors and developing muscle fibers can directly communicate through junctional channels. This review summarizes current knowledge on the GJIC and developmental events involved in the formation of skeletal muscle fibers and describes recent progress in the investigation of the role of GJIC in myogenesis: evidence of gap junctions in somitic and myotomal tissue as well as in developing muscle fibers in situ, GJIC between perfusion myoblasts in culture, and involvement of GJIC in cytodifferentiation of skeletal muscle cells and in myoblast fusion. A model of intercellular signaling is proposed where GJIC participates to coordinate a multicellular population of interacting myogenic precursors to allow commitment to the skeletal muscle fate.

Dendritic cell dynamics in the liver and hepatic lymph.

Abstract: Dendritic cells (DC) are bone-marrow-derived cells that function as professional antigen-presenting cells (APC). Liver is an essential organ for a host defense. It not only is armed with a powerful macrophage system but also is constantly surveyed by a heavy traffic of DC and lymphocytes. In case of emergency, such as infection and inflammation, DC traffic in the liver is accelerated. DC in the liver (interstitial DC) capture and process antigens, enter the draining lymph (DC in hepatic lymph) and accumulate in the T-cell area of hepatic lymph nodes (LN). DC in the LN present antigens to T and B cells to initiate immune responses. In accelerated states, DC precursors are recruited to the liver and soon translocate to hepatic lymph. Even mature lymph DC can undergo a blood-lymph translocation from the liver to hepatic LN after i.v. injection into normal rats. Rat Kupffer cells in the hepatic sinusoids are capable of selectively trapping DC from the blood in vivo and in vitro, suggesting involvement of certain adhesion molecules. Kupffer cells presumably elaborate chemokines to attract and trap the recruited DC via selective adhesion, leading to DC extravasation. The accelerated traffic and the presence of blood-lymph translocation would induce rapid and efficient immune responses and thus contribute to the local defense to antigens within liver tissues as well as systemic defense to blood-borne antigens. DC progenitors are also present in the liver, and these may play an important role in tolerance induction in liver transplantation.

Mechanisms of early neural crest development: from cell specification to migration.

Abstract: The neural crest is a group of embryonic progenitors that forms during the process of neurulation by interactions that take place between the prospective epidermis and the specified neuroectoderm. Although initially an integral part of the neuroepithelium, neural crest cells separate from the central nervous system primordium by a process of epitheliomesenchymal transition and become a motile cell population. These mesenchymal cells then migrate through stereotypic pathways, some of which are common and others unique to various vertebrate species. Furthermore, the availability of distinct migratory pathways also differs according to embryonic stage and axial level. Studies have begun to address the molecular basis of neural crest specification, delamination, and migration. The present review summarizes some major advances in our understanding of the nature of the intercellular interactions and the molecules that mediate them during early phases of neural crest ontogeny.

Cell biology of peroxisomes and their characteristics in aquatic organisms.

Abstract: The general characteristics of peroxisomes in different organisms, including aquatic organisms such as fish, crustaceans, and mollusks, are reviewed, with special emphasis on different aspects of the organelle biogenesis and mechanistic aspects of peroxisome proliferation. Peroxisome proliferation and peroxisomal enzyme inductions elicited by xenobiotics or physiological conditions have become useful tools to study the mechanisms of peroxisome biogenesis. During peroxisome proliferation, the induction of peroxisomal proteins is heterogeneous, enzymes that show increased activity being involved in different aspects of lipid homeostasis. The process of peroxisome biogenesis is coordinately triggered by a whole array of structurally dissimilar compounds known as peroxisome proliferators, and investigating the effect of some of these compounds that commonly appear as pollutants in the environment on the peroxisomes of aquatic animals inhabiting marine and estuarine habitats seems interesting. It is also important to determine whether peroxisome proliferation in these animals is a phenomenon that might occur under normal physiological or season-related conditions and plays a metabolic or functional role. This would help set the basis for understanding the process of peroxisome biogenesis in aquatic animals.

Cellular and molecular biology of capacitation and acrosome reaction in spermatozoa.

Abstract: A comparative account is given of advances in cellular and molecular biology of capacitation and acrosome reaction in spermatozoa by comparing and contrasting their biochemical and physiological changes in response to various factors in vivo and in vitro. It can now be stated that phenomena of sperm capacitation and acrosome reaction are endogenous molecular events occurring at the membrane level which can be modulated by external environmental factors. The molecular mechanisms and the signal transduction pathways mediating the process of capacitation and acrosome reaction are only partially defined and appear to involve modification of intracellular Ca2+ and other ions, lipid transfer, and phospholipid remodeling in the sperm plasma membrane as well as changes in protein phosphorylation. Evidences for the involvement of cAMP-dependent kinase pathway in the acrosome reaction are discussed. The mediation of one or more external signals by the sperm plasma membrane appears to activate this pathway after or simultaneously with the influx of Ca2+. Concurrent with or following entry of Ca2+, adenylate cyclase is activated, leading to increased concentrations of cAMP-activation of cAMP-dependent kinase and protein phosphorylation; the identity of such proteins and their role in the acrosome reaction must be determined. The roles of biological effectors of the acrosome reaction, such as ZP3 and follicular fluid are still to be defined at the molecular level. The gaps in our knowledge about the cellular and molecular aspects of capacitation and acrosome reaction are emphasized.

The Chlorella hexose/H(+)-symporters.

Abstract: The physiology, molecular biology, and biochemistry of the inducible hexose uptake protein of Chlorella kessleri is reviewed. The protein encoded by the HUP1 gene is the most intensively studied membrane transporter of plants. Responsible for substrate accumulation up to 1500-fold, it translocates one proton together with one hexose, and the cell invests 1 ATP per sugar transported. Kinetics suggest that substrate accumulation is mainly brought about by a large delta Km (Kminside >> Kmoutside). The HUP1 protein (534aa) consists of 12 transmembrane helices of which at least helices I, V, VII, and XI interact with the sugar during translocation and participate in lining the transport path through the membrane. The helix packing might very well be identical to the one suggested for the E. coli lac permease, although the mechanism for transport and proton coupling that has been suggested for lac permease (Kaback, 1997) certainly does not hold for the Chlorella symporter; both are distantly related members, however, of the MFS-family of transporters. HUP1 has been functionally expressed in Schizosaccharomyces pombe, Saccharomyces cerevisiae, Escherichia coli, Volvox carteri, and in Xenopus oocytes.

Partitioning and concentrating biomaterials in aqueous phase systems.

Abstract: Aqueous phase separation is a general phenomenon which occurs when structurally distinct water-soluble macromolecules are dissolved, above certain concentrations, in water. The number of aqueous phases obtained depends on the number of such distinct macromolecular species used. Aqueous two-phase systems, primarily those containing poly(ethylene glycol) and dextran, have been widely used for the separation of biomaterials (macromolecules, membranes, organelles, cells) by partitioning. The polymer and salt compositions and concentrations chosen greatly affect the physical properties of the phases. These, in turn, interact with the physical properties of biomaterials included in the phases and affect their partitioning. Specific extractions of biomaterials can be effected by including affinity ligands in the systems. The phase systems can also be used to obtain information on the surface properties of materials partitioned in them; to study interactions between biomaterials; and to concentrate such materials.

Regulation and expression of metazoan unconventional myosins.

Abstract: Unconventional myosins are molecular motors that convert adenosine triphosphate (ATP) hydrolysis into movement along actin filaments. On the basis of primary structure analysis, these myosins are represented by at least 15 distinct classes (classes 1 and 3–16), each of which is presumed to play a specific cellular role. However, in contrast to the conventional myosins-2, which drive muscle contraction and cytokinesis and have been studied intensively for many years in both uni- and multicellular organisms, unconventional myosins have only been subject to analysis in metazoan systems for a short time. Here we critically review what is known about unconventional myosin regulation, function, and expression. Several points emerge from this analysis. First, in spite of the high relative conservation of motor domains among the myosin classes, significant differences are found in biochemical and enzymatic properties of these motor domains. Second, the idea that characteristic distributions of unconventional myosins are solely dependent on the myosin tail domain is almost certainly an oversimplication. Third, the notion that most unconventional myosins function as transport motors for membranous organelles is challenged by recent data. Finally, we present a scheme that clarifies relationships between various modes of myosin regulation.

Factors controlling lineage specification in the neural crest.

Abstract: The neural crest is a transitory tissue of the vertebrate embryo that originates in the neural folds, populates the embryo, and gives rise to many different cell types and tissues of the adult organism. When neural crest cells initiate their migration, a large fraction of them are still pluripotent, that is, capable of generating progeny that consists of two or more distinct phenotypes. To elucidate the cellular and molecular mechanisms by which neural crest cells become committed to a particular lineage is therefore crucial to the understanding of neural crest development and represents a major challenge in current neural crest research. This chapter discusses selected aspects of neural crest cell differentiation into components of the peripheral nervous system. Topics include sympathetic neurons, the adrenal medulla, primary sensory neurons of the spinal ganglia, some of their mechanoreceptive and proprioceptive end organs, and the enteric nervous system.

Protooncogenes as mediators of apoptosis.

Abstract: Apoptosis has been well established as a vital biological phenomenon that is important in the maintenance of cellular homeostasis. Three major protooncogene families and their encoded proteins function as mediators of apoptosis in various cell types and are the subject of this chapter. Protooncogenic proteins such as c-Myc/Max, c-Fos/c-Jun, and Bcl-2/Bax utilize a synergetic effect to enhance their roles in the pro- or antiapoptotic action. These family members activate and repress the expression of their target genes, control cell cycle progression, and execute programmed cell death. Repression or overproduction of these protooncogenic proteins induces apoptosis, which may vary as a result of either cell type specificity or the nature of the apoptotic stimuli. The proapoptotic and antiapoptotic proteins exert their effects in the membrane of cellular organelles. Here they generate cell-type-specific signals that activate the caspase family of proteases and their regulators for the execution of apoptosis.

Genetic analysis of plant morphogenesis in vitro.

Abstract: Plant morphogenesis in vitro such as somatic embryogenesis and adventitious organogenesis has provided useful systems for physiological, biochemical, and molecular biological studies on plant development. Advantages of the in vitro systems are currently being combined with genetic techniques to generate new insights into fundamental mechanisms of plant development. This article presents an overview of genetic analysis of plant morphogenesis in vitro including genetic variation of tissue culture responses, mutational analysis of somatic embryogenesis, and mutational analysis of adventitious organogenesis.

Some aspects of plant karyology and karyosystematics.

Abstract: The significance of the 4C value (where C is the amount of DNA in the unreplicated haploid genome) in angiosperm plants is discussed. The DNA amount is a stable feature used in biosystematics. Although this parameter varies even in closely related taxa, there is no correlation between the DNA amount and the structural and functional organization of plants. The role of DNA amount, including "excess" DNA, in plant evolution is considered. Some rules governing the distribution of DNA amount among different plant taxa are postulated, together with the possibility of using the data in systematics, phylogeny, and solutions of problems of genetic apparatus organization and evolution. The decrease in DNA value per genome during plant evolution and the high level of species formation in taxa with large DNA values have been shown. Plant taxa with a small DNA value per genome have a high percentage and higher degree of polyploidy. The nature of the differential staining of euchromatin and heterochromatin bands of prophase and metaphase chromosomes is also discussed. Data that could explain the mechanism of heterochromatin visualization under cold pretreatment of cells are reviewed. Phenomena involved in the arrangement of chromocenters in interphase nuclei and chromosomes in metaphase during consecutive cell generations are discussed.

Macromolecular crowding and its consequences.

Abstract: Incompatible pairs of polymers separate into two phases in aqueous solution above a few percentage points total concentration. Protein pairs can also produce phase separation, but at somewhat higher concentrations. In this chapter, we explore the effect of high background concentrations of macromolecules on phase separation of pairs of species which would not be at sufficiently high concentration to separate in the absence of the uninvolved species. Effects produced by such high background concentrations are known as macromolecular crowding. Dramatic enhancements in various association reactions due to crowding have been predicted and observed but its effects on phase separation in biological mixtures typical of the cytoplasm have not been examined. Here, we describe a calculation based on the Flory-Huggins treatment of concentrated polymer solutions that sheds some light on this issue. We find that a background of 20 wt % of a high molecular weight species greatly reduces the concentrations needed to produce phase separation in a mixture of two incompatible macromolecules if one is more hydrophobic than the other. Given the high total concentration of macromolecules in cytoplasm, it is perhaps surprising that phases have not been observed. This issue is discussed and some explanations offered.