Claudin-based barrier in simple and stratified cellular sheets

For homeostasis in multicellular organisms, isolation and compartmentalisation of the internal environment are essential, and are established by various cellular sheets. For these cellular sheets to function as barriers, the intercellular route must be sealed. Recent advances reveal that claudins - major cell adhesion molecules in tight junctions - are directly involved in this intercellular sealing, not only in simple but also in stratified cellular sheets in vertebrates.     

Active Prestress Leads to an Apparent Stiffening of Cells through Geometrical Effects

Tuning of active prestress, e.g., through activity of molecular motors, constitutes a powerful cellular tool to adjust cellular stiffness through nonlinear material properties. Understanding this tool is an important prerequisite for our comprehension of cellular force response, cell shape dynamics, and tissue organization. Experimental data obtained from cell-mechanical measurements often show a simple linear dependence between mechanical prestress and measured differential elastic moduli. Although these experimental findings could point to stress-induced structural changes in the material, we propose a surprisingly simple alternative explanation in a theoretical study. We show how geometrical effects can give rise to increased cellular force response of cells in the presence of active prestress. The associated effective stress-stiffening is disconnected from actual stress-induced changes of the elastic modulus, and should therefore be regarded as an apparent stiffening of the material. We argue that new approaches in experimental design are necessary to separate this apparent stress-stiffening due to geometrical effects from actual nonlinearities of the elastic modulus in prestressed cellular material.     

Calculation of effective diffusivities for biofilms and tissues.

In this study we describe a scheme for numerically calculating the effective diffusivity of cellular systems such as biofilms and tissues. This work extends previous studies in which we developed the macroscale representations of the transport equations for cellular systems based on the subcellular-scale transport and reaction processes. A finite-difference model is used to predict the effective diffusivity of a cellular system on the basis of the subcellular-scale geometry and transport parameters. The effective diffusivity is predicted for a complex three-dimensional structure that is based on laboratory observations of a biofilm, and these numerical predictions are compared with predictions from a simple analytical solution and with experimental data. Our results indicate that, under many practical circumstances, the simple analytical solution can be used to provide reasonable estimates of the effective diffusivity.     

Exploring simple independent action in multifactor tables of proportions

The problem of assessing synergistic or antagonistic departure from simple independent action in multifactor tables of proportions is discussed. A generalized linear model is employed in which additivity corresponds to simple independent action. Data-analytic strategies are proposed for exploring departures from simple independent action in various extensions of the 2 X 2 table of proportions. This methodology is illustrated with a series of models fitted to cellular differentiation and murine toxicity data.     

A simple and reliable method for the determination of cellular RNA content

Summary This communication describes a rapid, simple and reliable method for the determination of the cellular RNA content. In the study of microbial growth and product formation the cellular RNA content is a good measure of the activity of the biomass. Results from applying the method to monitor transient experiments in a chemostat with Lactococcus cremoris are presented.     

The life of lipid droplets

Lipid droplets are the least characterized of cellular organelles. Long considered simple lipid storage depots, these dynamic and remarkable organelles have recently been implicated in many biological processes, and we are only now beginning to gain insights into their fascinating lives in cells. Here we examine what we know of the life of lipid droplets. We review emerging data concerning their cellular biology and present our thoughts on some of the most salient questions for investigation.     

Can mechanics control pattern formation in plants?

Development of the plant body entails many pattern forming events at scales ranging from the cellular level to the whole plant. Recent evidence suggests that mechanical forces play a role in establishing some of these patterns. The development of cellular configurations in glandular trichomes and the rippling of leaf surfaces are discussed in depth to illustrate how intricate patterns can emerge from simple and well-established molecular and cellular processes. The ability of plants to sense and transduce mechanical signals suggests that complex interactions between mechanics and chemistry are possible during plant development. The inclusion of mechanics alongside traditional molecular controls offers a more comprehensive view of developmental processes.     

Dynamic cellular automata: an alternative approach to cellular simulation

A wide variety of approaches, ranging from Petri nets to systems of partial differential equations, have been used to model very specific aspects of cellular or biochemical functions. Here we describe how an agent-based or dynamic cellular automata (DCA) approach can be used as a very simple, yet very general method to model many different kinds of cellular or biochemical processes. Specifically, using simple pairwise interaction rules coupled with random object moves to simulate Brownian motion, we show how the DCA approach can be used to easily and accurately model diffusion, viscous drag, enzyme rate processes, metabolism (the Kreb's cycle), and complex genetic circuits (the repressilator). We also demonstrate how DCA approaches are able to accurately capture the stochasticity of many biological processes. The success and simplicity of this technique suggests that many other physical properties and significantly more complicated aspects of cellular behavior could be modeled using DCA methods. An easy-to-use, graphically-based computer program, called SimCell, was developed to perform the DCA simulations described here. It is available at http://wishart.biology.ualberta.ca/SimCell/.     

Extended disordered proteins: targeting function with less scaffold

It has been estimated that a large fraction of cellular proteins are natively disordered. Current opinion largely holds that natively disordered proteins are more 'adaptive', leading to advantages in regulation and in binding diverse ligands. Here, we argue for another, simple, physically based reason. Disordered proteins often have large intermolecular interfaces, the size of which is dictated by protein function. For proteins to be stable as monomers with extensive interfaces, protein size would need to be 2-3 times larger. This would either increase cellular crowding or enlarge the size of the cell by 15-30%, owing to the increase in the sequence length. Smaller sizes of cells, proteins, DNA and RNA conserve energy. Thus, disordered proteins provide a simple yet elegant solution to having large intermolecular interfaces, but with smaller protein, genome and cell sizes.     

de Broglie waves in amoeboid motility.

The de Broglie wave equation has been applied to the study of amoeboid motility. This leads to precise predictions of wavelengths displayed by the cellular membrane during motility. Motile amoeba are compared to non-biological systems showing de Broglie wave behavior, and three simple experiments are suggested.     

Molecular machinery of mitochondrial dynamics in yeast

Mitochondria are amazingly dynamic organelles. They continuously move along cytoskeletal tracks and frequently fuse and divide. These processes are important for maintenance of mitochondrial functions, for inheritance of the organelles upon cell division, for cellular differentiation and for apoptosis. As the machinery of mitochondrial behavior has been highly conserved during evolution, it can be studied in simple model organisms, such as yeast. During the past decade, several key components of mitochondrial dynamics have been identified and functionally characterized in Saccharomyces cerevisiae. These include the mitochondrial fusion and fission machineries and proteins required for maintenance of tubular shape and mitochondrial motility. Taken together, these findings reveal a comprehensive picture that shows the cellular processes and molecular components required for mitochondrial inheritance and morphogenesis in a simple eukaryotic cell.     

细菌必需基因、最小基因组和合成细胞

单细胞原核生物是原始的细胞生命形式,确定细菌必需基因和最小基因组对理解生命的本质、细胞生命的起源和进化有非常重要的意义。文中简要介绍近年来有关细菌的必需基因、最小基因组和合成细胞的研究方法、理论和进展。还特别介绍人工建立最小细菌基因组的策略以及应用前景。     

Contributions to the mathematical theory of organic form. II Asymmetric metabolism of cellular aggregates

The case is studied of a cellular aggregate forming a hollow spherical shell, the physical constants and the metabolic rate being non-uniform throughout the shell. This leads to asymmetries of concentration distributions, which are calculated here for some simple cases.     

Computational model of flow-tissue interactions in intussusceptive angiogenesis

Angiogenesis, the growth of vascular structures, is a complex biological process which has long puzzled scientists. Better physiological understanding of this phenomenon could result in many useful medical applications such as the development of new methods for cancer therapy. We report on the development of a simple computational model of micro-vascular structure formation in intussusceptive angiogenesis observed in vivo. The tissue is represented by a discrete set of basic structural entities and flow conditions within the resulting domain are obtained by solving the Navier-Stokes equations. The tissue is then remodelled according to the tangential shear stress while approximating advection by means of simple non-diffusive heuristics. The updated tissue geometry then becomes the input for the next remodelling step. The model, consisting of steady-state flow and a simple mechanistic tissue response, successfully predicts bifurcation formation and micro-vessel separation in a porous cellular medium. This opens new modelling possibilities in computational studies of the cellular transport involved in micro-vascular growth.     

Programming microbial population dynamics by engineered cell-cell communication

A major aim of synthetic biology is to program novel cellular behavior using engineered gene circuits. Early endeavors focused on building simple circuits that fulfill simple functions, such as logic gates, bistable toggle switches, and oscillators. These gene circuits have primarily focused on single-cell behaviors since they operate intracellularly. Thus, they are often susceptible to cell-cell variations due to stochastic gene expression. Cell-cell communication offers an efficient strategy to coordinate cellular behavior at the population level. To this end, we review recent advances in engineering cell-cell communication to achieve reliable population dynamics, spanning from communication within single species to multispecies, from one-way sender-receiver communication to two-way communication in synthetic microbial ecosystems. These engineered systems serve as well-defined model systems to better understand design principles of their naturally occurring counterparts and to facilitate novel biotechnology applications.     

Simple methods which maintain the barrier status of specific-pathogen-free animals during experimentation

Several simple procedures for experimentation on specific-pathogen-free rats and mice are described which combine the technique of plastic-film-isolator containment and a laminar flow sterile environment. These permit the full range of cellular immunology experiments to be performed without compromising the microbiological barrier.     

用FCM双参数测量膜抗原及核DNA的技术关键

利用流式细胞仪同时定量测定细胞核DNA及细胞膜表面抗原．对测量原理、条件、各种误差的消除方法及可行性进行了阐述.建立了一种简便易行的测量与分析方法．此方法已被应用于多项研究工作中.     

Roles for host factors in plant viral pathogenicity

The simple, obligate nature of viruses requires them to usurp or divert cellular resources, including host factors, away from their normal functions. The characterization of host proteins, membranes, and nucleic acids that are implicated in viral infection cycles, together with other recent discoveries, is providing fundamental clues about the molecular bases of viral susceptibility. As viruses invade susceptible plants, they create conditions that favor systemic infections by suppressing multiple layers of innate host defenses. When viruses meddle in these defense mechanisms, which are interlinked with basic cellular functions, phenotypic changes can result that contribute to disease symptoms.     

Building the cell: design principles of cellular architecture

The astounding structural complexity of a cell arises from the action of a relatively small number of genes, raising the question of how this complexity is achieved. Self-organizing processes combined with simple physical constraints seem to have key roles in controlling organelle size, number, shape and position, and these factors then combine to produce the overall cell architecture. By examining how these parameters are controlled in specific cell biological examples we can identify a handful of simple design principles that seem to underlie cellular architecture and assembly.