Dissection of cell division processes in the one cell stage Caenorhabditis elegans embryo by mutational analysis

To identify novel components required for cell division processes in complex eukaryotes, we have undertaken an extensive mutational analysis in the one cell stage Caenorhabditis elegans embryo. The large size and optical properties of this cell permit observation of cell division processes with great detail in live specimens by simple differential interference contrast (DIC) microscopy. We have screened an extensive collection of maternal-effect embryonic lethal mutations on chromosome III with time-lapse DIC video microscopy. Using this assay, we have identified 48 mutations in 34 loci which are required for specific cell division processes in the one cell stage embryo. We show that mutations fall into distinct phenotypic classes which correspond, among others, to the processes of pronuclear migration, rotation of centrosomes and associated pronuclei, spindle assembly, chromosome segregation, anaphase spindle positioning, and cytokinesis. We have further analyzed pronuclear migration mutants by indirect immunofluorescence microscopy using antibodies against tubulin and ZYG-9, a centrosomal marker. This analysis revealed that two pronuclear migration loci are required for generating normal microtubule arrays and four for centrosome separation. All 34 loci have been mapped by deficiencies to distinct regions of chromosome III, thus paving the way for their rapid molecular characterization. Our work contributes to establishing the one cell stage C. elegans embryo as a powerful metazoan model system for dissecting cell division processes.     

The cell biology of aging

One of the original hypotheses of organismal longevity posits that aging is the natural result of entropy on the cells, tissues, and organs of the animal—a slow, inexorable slide into nonfunctionality caused by stochastic degradation of its parts. We now have evidence that aging is instead at least in part genetically regulated. Many mutations have been discovered to extend lifespan in organisms of all complexities, from yeast to mammals. The study of metazoan model organisms, such as Caenorhabditis elegans, has been instrumental in understanding the role of genetics in the cell biology of aging. Longevity mutants across the spectrum of model organisms demonstrate that rates of aging are regulated through genetic control of cellular processes. The regulation and subsequent breakdown of cellular processes represent a programmatic decision by the cell to either continue or abandon maintenance procedures with age. Our understanding of cell biological processes involved in regulating aging have been particularly informed by longevity mutants and treatments, such as reduced insulin/IGF-1 signaling and dietary restriction, which are critical in determining the distinction between causes of and responses to aging and have revealed a set of downstream targets that participate in a range of cell biological activities. Here we briefly review some of these important cellular processes.     

Regulation of Par6 by extracellular signals

The critical regulator of polarity, Par6, is a key member of a multi-component polarity complex that controls a variety of cellular processes such as asymmetric cell division, establishment of epithelial apico-basal polarity, and polarized cell migration. Recently, we have come to understand how regulation of the Par6 interactome by extracellular cues such as integrin and transforming growth factor beta signalling regulates cell motility and tight junction dissolution. These studies have begun to elucidate how signalling to the polarity complex might regulate pathological processes such as tumour cell invasion and metastasis.     

Vertex Models of Epithelial Morphogenesis

The dynamic behavior of epithelial cell sheets plays a central role during numerous developmental processes. Genetic and imaging studies of epithelial morphogenesis in a wide range of organisms have led to increasingly detailed mechanisms of cell sheet dynamics. Computational models offer a useful means by which to investigate and test these mechanisms, and have played a key role in the study of cell-cell interactions. A variety of modeling approaches can be used to simulate the balance of forces within an epithelial sheet. Vertex models are a class of such models that consider cells as individual objects, approximated by two-dimensional polygons representing cellular interfaces, in which each vertex moves in response to forces due to growth, interfacial tension, and pressure within each cell. Vertex models are used to study cellular processes within epithelia, including cell motility, adhesion, mitosis, and delamination. This review summarizes how vertex models have been used to provide insight into developmental processes and highlights current challenges in this area, including progressing these models from two to three dimensions and developing new tools for model validation.     

Vertex Models of Epithelial Morphogenesis

The dynamic behavior of epithelial cell sheets plays a central role during numerous developmental processes. Genetic and imaging studies of epithelial morphogenesis in a wide range of organisms have led to increasingly detailed mechanisms of cell sheet dynamics. Computational models offer a useful means by which to investigate and test these mechanisms, and have played a key role in the study of cell-cell interactions. A variety of modeling approaches can be used to simulate the balance of forces within an epithelial sheet. Vertex models are a class of such models that consider cells as individual objects, approximated by two-dimensional polygons representing cellular interfaces, in which each vertex moves in response to forces due to growth, interfacial tension, and pressure within each cell. Vertex models are used to study cellular processes within epithelia, including cell motility, adhesion, mitosis, and delamination. This review summarizes how vertex models have been used to provide insight into developmental processes and highlights current challenges in this area, including progressing these models from two to three dimensions and developing new tools for model validation.     

Fine structure of taste buds in the barbel of the catfish,Ictalurus punctatus

Summary Taste buds occur in the epithelium of the catfish barbel along its entire length. Two major cell types, light and dark cells, occupy the upper two-thirds of the taste bud. A third cell type, the basal cell, lies on the basal lamina and is essentially separated from the light and dark cells by a plexus of unmyelinated nerve fibers. The dark cells have branching processes, both apically and basally whereas the light cells have a single apical process and many basal processes. The apical processes of dark cells contain secretory granules, while the apical processes of light cells contain an abundant agranular endoplasmic reticulum. Light cell nuclei contain bundles of 10 nm filaments, often arranged in the shape of a cup or ring, but nucleoli are rarely seen. It is suggested that this morphology indicates a low degree of RNA synthesis by light cells. The basal cells contain large numbers of vesicles, about 60 nm in diameter, which are sometimes seen in clumps in relation to an adjacent nerve fiber in a configuration resembling a synapse. Curiously, although basal cells present a large surface to the basal lamina, there are no hemidesmosomes. This suggests that the basal cell does not originate from the epidermis.Supported by grant#NS-06181 from the National Institute of Neurological Diseases and Stroke, U.S. Public Health Service     

An abstract cell model that describes the self-organization of cell function in living systems

The principal aim of systems biology is to search for general principles that govern living systems. We develop an abstract dynamic model of a cell, rooted in Mesarovi? and Takahara's general systems theory. In this conceptual framework the function of the cell is delineated by the dynamic processes it can realize. We abstract basic cellular processes, i.e., metabolism, signalling, gene expression, into a mapping and consider cell functions, i.e., cell differentiation, proliferation, etc. as processes that determine the basic cellular processes that realize a particular cell function. We then postulate the existence of a 'coordination principle' that determines cell function. These ideas are condensed into a theorem: If basic cellular processes for the control and regulation of cell functions are present, then the coordination of cell functions is realized autonomously from within the system. Inspired by Robert Rosen's notion of closure to efficient causation, introduced as a necessary condition for a natural system to be an organism, we show that for a mathematical model of a self-organizing cell the associated category must be cartesian closed. Although the semantics of our cell model differ from Rosen's (M,R)-systems, the proof of our theorem supports (in parts) Rosen's argument that living cells have non-simulable properties. Whereas models that form cartesian closed categories can capture self-organization (which is a, if not the, fundamental property of living systems), conventional computer simulations of these models (such as virtual cells) cannot. Simulations can mimic living systems, but they are not like living systems.     

Intercellular contacts of astroglial cells in vitro in the presence of PGE2. SEM, TEM and immunocytochemical studies

The investigations were performed on 32 tumors of the CNS of fibrillary and gemistocyte type of astrocytoma from which the cultures were derived. The dissociated culture was used and the cells were seeded direct on glass. The MEM was supplemented with calf serum and embryonal extract. The cells maintained in culture for 21 days. They developed multiple or single long and delicate processes which originated a network of relative high density. SEM observations indicate that the cell established intercellular connections through spikes and surfaces of the processes and cell bodies. TEM studies have shown that the cell to cell connections are very tight but the contacts between the cell processes are similar to those described by DUFFY as punctate connections. The addition of PGE2 does not disturb the reconstruction of intercellular communication. The estimation of GFAP antigenicity of the investigated cells proved their astrocytic origin and revealed that the intermediate fibres are in the cell processes, and in the location of the contacts between cells.     

A computational biomimetic study of cell crawling

Cell locomotion is a result of a series of synchronized chemo-mechanical processes. Previous extensive experimental studies have revealed many chemo-mechanical processes that may contribute to cell locomotion. In parallel, theoretical works have been developed to provide deeper insight. To date, however, direct simulations of cell locomotion on a substrate have not been seen. In this paper, a finite element–based computational model is developed to study amoeboid type of cell crawling phenomenon. Here, a cell is modeled as a 2D fluid-filled elastic vesicle, which establishes its interaction with a rigid substrate through a kinetics-based cellular adhesion model. The cell derives its motion through a differential bond breaking at the trailing edge and bond formation at the leading edge. This mechanism of crawling authenticates the hypothesis that cell locomotion can be facilitated by breaking the adhesive bonds at the rear edge, which was initially proposed by Chen (J Cell Biol 90: 187–200, 1981).     

Regulation of epithelial cell shape and polarity by cell - cell adhesion (Review)

Among all cell types that exhibit a polarized phenotype, epithelial cells are unique in that their polarity depends on the integration of the cell into a tissue, the epithelium. In recent years, the analysis of epithelial cell polarity in different epithelia and organisms has contributed to an understanding of the components involved and has further demonstrated that cell polarity and cell adhesion are intimately related to each other. Therefore, processes that mediate and modulate cell adhesion and coordinate adhesion and cell shape are fundamental for the function of epithelia. Recent results obtained in Drosophila melanogaster and Caenorhabditis elegans have provided further insight into the complex circuits regulating these processes, and have laid the direction for future analysis.     

Regulation of epithelial cell shape and polarity by cell-cell adhesion (Review)

Among all cell types that exhibit a polarized phenotype, epithelial cells are unique in that their polarity depends on the integration of the cell into a tissue, the epithelium. In recent years, the analysis of epithelial cell polarity in different epithelia and organisms has contributed to an understanding of the components involved and has further demonstrated that cell polarity and cell adhesion are intimately related to each other. Therefore, processes that mediate and modulate cell adhesion and coordinate adhesion and cell shape are fundamental for the function of epithelia. Recent results obtained in Drosophila melanogaster and Caenorhabditis elegans have provided further insight into the complex circuits regulating these processes, and have laid the direction for future analysis.     

Calcium response in single osteocytes to locally applied mechanical stimulus: Differences in cell process and cell body

It is proposed that osteocytes embedded in the bone matrix have the ability to sense deformation and/or damage to the matrix and to feed these mechanical signals back to the adaptive bone remodeling process. When osteoblasts differentiate into osteocytes during the bone formation process, they change their morphology to a stellate form with many slender processes. This characteristic cell shape may underlie the differences in mechanosensitivity between the cell processes and cell body. To elucidate the mechanism of cellular response to mechanical stimulus in osteocytes, we investigated the site-dependent response to quantitatively controlled local mechanical stimulus in single osteocytes isolated from chick embryos, using the technique of calcium imaging. A mechanical stimulus was applied to a single osteocyte using a glass microneedle targeting a microparticle adhered to the cell membrane by modification with a monoclonal antibody OB7.3. Application of the local deformation induced calcium transients in the vicinity of the stimulated point and caused diffusive wave propagation of the calcium transient to the entire intracellular region. The rate of cell response to the stimulus was higher when applied to the cell processes than when applied to the cell body. In addition, a large deformation was necessary at the cell body to induce calcium transients, whereas a relatively small deformation was sufficient at the cell processes, suggesting that the mechanosensitivity of the cell processes was higher than that of the cell body. These results suggest that the cell shape with slender processes contributes to the site-dependent mechanosensitivity in osteocytes.     

The development of the plasma membrane reticular system in the fat body of an insect

Several insect tissues have plasma membranes that are folded inwards to make a subsurface reticulum on faces that are exposed to hemolymph. The infolds have been called plasma membrane reticular systems (RSs) to distinguish them from the somewhat similar structures found in transporting epithelia. They are characterized by having negative charges on the plasma membranes of the entranceways and by the concentration of some hemolymph proteins in their lymph spaces. Their formation and loss in the fat body has been studied by scanning electron microscopy during the fifth stadium of Calpodes ethlius (Lepidoptera, Hesperiidae). Fat body cells begin the fifth stadium arranged in ribbons with the cells linked together by a fringe of processes. In the first stage many more processes form. These partially fuse together in the second stage, leaving a subsurface reticulum connected by narrow entrances to the lateral cell faces and the face below the basal lamina. Both the cell processes and the reticular systems that they enclose are usually axially orientated. The completed RS persists for the second half of the intermoult devoted to larval syntheses when the concentration of hemolymph proteins rises. After protein sequestration prior to pupation the RS is lost and the fat body returns to being a tissue of rounded cells linked by a few enmeshed processes.     

The effects of vitamin D on skeletal muscle function and cellular signaling

It is thought that every cell in the body expresses the vitamin D receptor, and therefore vitamin D may play a role in health and homeostasis of every organ system, including skeletal muscle. Human, animal, and cell culture studies have collectively shown that vitamin D affects muscle strength and function. Vitamin D functions in a plethora of cellular processes in skeletal muscle including calcium homeostasis, cell proliferation, cell differentiation, fiber size, prevention of fatty degeneration, protection against insulin resistance and arachidonic acid mobilization. These processes appear to be mediated by several signaling pathways affected by vitamin D. This review aims to explore the effects of vitamin D on skeletal muscle in each model system and to delineate potential cell signaling pathways affected by vitamin D.     

Developmental roles of platelet-derived growth factors

Platelet-derived growth factor (PDGF) was originally identified in platelets and in serum as a mitogen for fibroblasts, smooth muscle cells (SMC) and glia cells in culture. PDGF has since expanded to a family of dimers of at least four gene products, whose biological actions are mediated through two receptor tyrosine kinases, PDGFRs. The present review summarizes and discusses the biological functions of PDGFs and PDGFRs in developmental processes, mainly as revealed through genetic analysis in mice. Such studies have demonstrated multiple critical roles of PDGFs and PDGFRs in embryonic and postnatal development. PDGFs seem to act upon specific populations of progenitor cells that give rise to several different cell types with distinct functions in a variety of developmental processes. Analogies are seen between the cell functions and the developmental processes controlled by PDGFs. This suggests that ancestral PDGF and PDGFR expression patterns and functions may have been iterated in related sets of morphogenetic processes in the course of evolution.     

Fine structure of the epidermis of the optic tentacle in a slug, Limax flavus L.

The epidermis at the tip of the optic tentacle in Limax flavus is constructed of columnar epithelial cells, distal processes of nerve cells, and scattered processes of the collar cells. The epithelial cells extend stout microvilli called plasmatic processes by Wright perpendicularly from the free surface. Each plasmic process branches into a few terminal twigs embedded in a fuzzy filamentous substance. Most nerve cells have their nuclei under the basal lamina. The distal processes of these nerve cells reach the free surface and send long microvilli to form the spongy layer under a filamentous covering. At the side surface of the tentacle the epithelial cells are cuboidal or squamous and the neural elements are fewer. Here, no spongy layer is formed; and the collar cell processes are replaced by the lateral cell processes. Peculiar secretion granules are contained in the lateral and collar cell processes as well as in their cell bodies situated beneath the basal lamina.     

The plot thickens: New perspectives of primary cell wall modification

Recent studies have further confirmed the ubiquity of cell wall restructuring during plant growth and development, and have emphasized the fact that our understanding of the breadth of molecular processes that mediate wall modification is still rudimentary. In the past few years, both enzymatic and non-enzymatic agents that apparently contribute to wall disassembly have been identified, and it is likely that additional mechanisms will continue to be revealed. These discoveries are being propelled by the development of new biochemical and biophysical assays, by database mining in the wake of the explosion of plant sequence information from genome sequencing and expressed sequence tags, and by a variety of strategies used to catalog the cell wall proteome. The daunting question of how these mechanistically diverse and complex processes are coordinated remains unresolved.     

PIN polarity maintenance by the cell wall in Arabidopsis

A central question in developmental biology concerns the mechanism of generation and maintenance of cell polarity, because these processes are essential for many cellular functions and multicellular development. In plants, cell polarity has an additional role in mediating directional transport of the plant hormone auxin that is crucial for multiple developmental processes. In addition, plant cells have a complex extracellular matrix, the cell wall, whose role in regulating cellular processes, including cell polarity, is unexplored. We have found that polar distribution of PIN auxin transporters in plant cells is maintained by connections between polar domains at the plasma membrane and the cell wall. Genetic and pharmacological interference with cellulose, the major component of the cell wall, or mechanical interference with the cell wall disrupts these connections and leads to increased lateral diffusion and loss of polar distribution of PIN transporters for the phytohormone auxin. Our results reveal a plant-specific mechanism for cell polarity maintenance and provide a conceptual framework for modulating cell polarity and plant development via endogenous and environmental manipulations of the cellulose-based extracellular matrix.     

Roles of VRK1 as a new player in the control of biological processes required for cell division

Cell division, in addition to an accurate transmission of genetic information to daughter cells, also requires the temporal and spatial coordination of several biological processes without which cell division would not be feasible. These processes include the temporal coordination of DNA replication and chromosome segregation, regulation of nuclear envelope disassembly and assembly, chromatin condensation and Golgi fragmentation for its redistribution into daughter cells, among others. However, little is known regarding regulatory proteins and signalling pathways that might participate in the coordination of all these different biological functions. Such regulatory players should directly have a role in the processes leading to cell division. VRK1 (Vaccinia-related kinase 1) is an early response gene required for cyclin D1 expression, regulates p53 by a specific Thr18 phosphorylation, controls chromatin condensation by histone phosphorylation, nuclear envelope assembly by phosphorylation of BANF1, and participates in signalling required for Golgi fragmentation late in the G2 phase. We propose that VRK1, a Ser-Thr kinase, might be a candidate to play an important coordinator role in these cell division processes as part of a novel signalling pathway.