22 research outputs found
An equilibrium model for ribosome competition
The number of ribosomes in a cell is considered as limiting, and gene
expression is thus largely determined by their cellular concentration. In this
work we develop a toy model to study the trade-off between the ribosomal supply
and the demand of the translation machinery, dictated by the composition of the
transcript pool. Our equilibrium framework is useful to highlight qualitative
behaviours and new means of gene expression regulation determined by the fine
balance of this trade-off. We also speculate on the possible impact of these
mechanisms on cellular physiology
Characterization of Intrinsic Properties of Promoters.
Accurate characterization of promoter behavior is essential for the rational design of functional synthetic transcription networks such as logic gates and oscillators. However, transcription rates observed from promoters can vary significantly depending on the growth rate of host cells and the experimental and genetic contexts of the measurement. Furthermore, in vivo measurement methods must accommodate variation in translation, protein folding, and maturation rates of reporter proteins, as well as metabolic load. The external factors affecting transcription activity may be considered to be extrinsic, and the goal of characterization should be to obtain quantitative measures of the intrinsic characteristics of promoters. We have developed a promoter characterization method that is based on a mathematical model for cell growth and reporter gene expression and exploits multiple in vivo measurements to compensate for variation due to extrinsic factors. First, we used optical density and fluorescent reporter gene measurements to account for the effect of differing cell growth rates. Second, we compared the output of reporter genes to that of a control promoter using concurrent dual-channel fluorescence measurements. This allowed us to derive a quantitative promoter characteristic (ρ) that provides a robust measure of the intrinsic properties of a promoter, relative to the control. We imposed different extrinsic factors on growing cells, altering carbon source and adding bacteriostatic agents, and demonstrated that the use of ρ values reduced the fraction of variance due to extrinsic factors from 78% to less than 4%. This is a simple and reliable method to quantitatively describe promoter properties.TJR was supported by a Microsoft Research studentship and EC FP7 Project No. 612146 (PLASWIRES) awarded to JH, JRB by a Microsoft Research studentship and internship, and FF by CONICYT-PAI/Concurso Nacional de Apoyo al Retorno de Investigadores/as desde el Extranjero Folio 8213002 7, and EPSRC grant EP/H019162/1 awarded to JH. JWA acknowledges the EPSRC and the Wellcome Trust for support.This is the author accepted manuscript. The final version is available from ACS via http://dx.doi.org/10.1021/acssynbio.5b0011
Orthogonal intercellular signaling for programmed spatial behavior.
Bidirectional intercellular signaling is an essential feature of multicellular organisms, and the engineering of complex biological systems will require multiple pathways for intercellular signaling with minimal crosstalk. Natural quorum-sensing systems provide components for cell communication, but their use is often constrained by signal crosstalk. We have established new orthogonal systems for cell-cell communication using acyl homoserine lactone signaling systems. Quantitative measurements in contexts of differing receiver protein expression allowed us to separate different types of crosstalk between 3-oxo-C6- and 3-oxo-C12-homoserine lactones, cognate receiver proteins, and DNA promoters. Mutating promoter sequences minimized interactions with heterologous receiver proteins. We used experimental data to parameterize a computational model for signal crosstalk and to estimate the effect of receiver protein levels on signal crosstalk. We used this model to predict optimal expression levels for receiver proteins, to create an effective two-channel cell communication device. Establishment of a novel spatial assay allowed measurement of interactions between geometrically constrained cell populations via these diffusible signals. We built relay devices capable of long-range signal propagation mediated by cycles of signal induction, communication and response by discrete cell populations. This work demonstrates the ability to systematically reduce crosstalk within intercellular signaling systems and to use these systems to engineer complex spatiotemporal patterning in cell populations.PKG acknowledges support from the John Templeton Foundation Grant ID#15619: “Mind, Mechanism and Mathematics: Turing Centenary Research Project”. JH acknowledges Biotechnology and Biological Sciences Research Council and Engineering and Physical Sciences Research Council (RG72490), and FF acknowledges support from CONICYT‐PAI/Concurso Nacional de Apoyo al Retorno de Investigadores/as desde el Extranjero Folio 82130027. We would like to thank J. Ajioka and O. Yarkoni for use of equipment and advice. We would like to thank P.J. Steiner for early discussions about this work
A computational framework for testing hypotheses of the minimal mechanical requirements for cell aggregation using early annual killifish embryogenesis as a model
Introduction: Deciphering the biological and physical requirements for the outset of multicellularity is limited to few experimental models. The early embryonic development of annual killifish represents an almost unique opportunity to investigate de novo cellular aggregation in a vertebrate model. As an adaptation to seasonal drought, annual killifish employs a unique developmental pattern in which embryogenesis occurs only after undifferentiated embryonic cells have completed epiboly and dispersed in low density on the egg surface. Therefore, the first stage of embryogenesis requires the congregation of embryonic cells at one pole of the egg to form a single aggregate that later gives rise to the embryo proper. This unique process presents an opportunity to dissect the self-organizing principles involved in early organization of embryonic stem cells. Indeed, the physical and biological processes required to form the aggregate of embryonic cells are currently unknown.Methods: Here, we developed an in silico, agent-based biophysical model that allows testing how cell-specific and environmental properties could determine the aggregation dynamics of early Killifish embryogenesis. In a forward engineering approach, we then proceeded to test two hypotheses for cell aggregation (cell-autonomous and a simple taxis model) as a proof of concept of modeling feasibility. In a first approach (cell autonomous system), we considered how intrinsic biophysical properties of the cells such as motility, polarity, density, and the interplay between cell adhesion and contact inhibition of locomotion drive cell aggregation into self-organized clusters. Second, we included guidance of cell migration through a simple taxis mechanism to resemble the activity of an organizing center found in several developmental models.Results: Our numerical simulations showed that random migration combined with low cell-cell adhesion is sufficient to maintain cells in dispersion and that aggregation can indeed arise spontaneously under a limited set of conditions, but, without environmental guidance, the dynamics and resulting structures do not recapitulate in vivo observations.Discussion: Thus, an environmental guidance cue seems to be required for correct execution of early aggregation in early killifish development. However, the nature of this cue (e.g., chemical or mechanical) can only be determined experimentally. Our model provides a predictive tool that could be used to better characterize the process and, importantly, to design informed experimental strategies
Synthetic Biology: opportunities for Chilean bioindustry and education
In an age of pressing challenges for sustainable production of energy and food, the new field of Synthetic Biology has emerged as a promising approach to engineer biological systems. Synthetic Biology is formulating the design principles to engineer affordable, scalable, predictable and robust functions in biological systems. In addition to efficient transfer of evolved traits from one organism to another, Synthetic Biology offers a new and radical approach to bottom-up engineering of sensors, actuators, dynamical controllers and the biological chassis they are embedded in. Because it abstracts much of the mechanistic details underlying biological component behavior, Synthetic Biology methods and resources can be readily used by interdisciplinary teams to tackle complex problems. In addition, the advent of robust new methods for the assembly of large genetic circuits enables teaching Biology and Bioengineering in a learning-by-making fashion for diverse backgrounds at the graduate, undergraduate and high school levels. Synthetic Biology offers unique opportunities to empower interdisciplinary training, research and industrial development in Chile for a technology that promises a significant role in this century's economy
Computational Modeling of Synthetic Microbial Biofilms
Microbial biofilms are complex, self-organized communities
of bacteria, which employ physiological cooperation and spatial organization
to increase both their metabolic efficiency and their resistance to
changes in their local environment. These properties make biofilms
an attractive target for engineering, particularly for the production
of chemicals such as pharmaceutical ingredients or biofuels, with
the potential to significantly improve yields and lower maintenance
costs. Biofilms are also a major cause of persistent infection, and
a better understanding of their organization could lead to new strategies
for their disruption. Despite this potential, the design of synthetic
biofilms remains a major challenge, due to the complex interplay between
transcriptional regulation, intercellular signaling, and cell biophysics.
Computational modeling could help to address this challenge by predicting
the behavior of synthetic biofilms prior to their construction; however,
multiscale modeling has so far not been achieved for realistic cell
numbers. This paper presents a computational method for modeling synthetic
microbial biofilms, which combines three-dimensional biophysical models
of individual cells with models of genetic regulation and intercellular
signaling. The method is implemented as a software tool (<i>CellModeller</i>), which uses parallel Graphics Processing Unit architectures to
scale to more than 30,000 cells, typical of a 100 μm diameter
colony, in 30 min of computation time
Computational Modeling of Synthetic Microbial Biofilms
Microbial biofilms are complex, self-organized communities
of bacteria, which employ physiological cooperation and spatial organization
to increase both their metabolic efficiency and their resistance to
changes in their local environment. These properties make biofilms
an attractive target for engineering, particularly for the production
of chemicals such as pharmaceutical ingredients or biofuels, with
the potential to significantly improve yields and lower maintenance
costs. Biofilms are also a major cause of persistent infection, and
a better understanding of their organization could lead to new strategies
for their disruption. Despite this potential, the design of synthetic
biofilms remains a major challenge, due to the complex interplay between
transcriptional regulation, intercellular signaling, and cell biophysics.
Computational modeling could help to address this challenge by predicting
the behavior of synthetic biofilms prior to their construction; however,
multiscale modeling has so far not been achieved for realistic cell
numbers. This paper presents a computational method for modeling synthetic
microbial biofilms, which combines three-dimensional biophysical models
of individual cells with models of genetic regulation and intercellular
signaling. The method is implemented as a software tool (<i>CellModeller</i>), which uses parallel Graphics Processing Unit architectures to
scale to more than 30,000 cells, typical of a 100 μm diameter
colony, in 30 min of computation time
Cell Polarity-Driven Instability Generates Self-Organized, Fractal Patterning of Cell Layers
As a model system to study physical
interactions in multicellular
systems, we used layers of <i>Escherichia coli</i> cells, which exhibit little or no intrinsic coordination
of growth. This system effectively isolates the effects of cell shape,
growth, and division on spatial self-organization. Tracking the development
of fluorescence-labeled cellular domains, we observed the emergence
of striking fractal patterns with jagged, self-similar shapes. We
then used a large-scale, cellular biophysical model to show that local
instabilities due to polar cell-shape, repeatedly propagated by uniaxial
growth and division, are responsible for generating this fractal geometry.
Confirming this result, a mutant of <i>E. coli</i> with
spherical shape forms smooth, nonfractal cellular domains. These results
demonstrate that even populations of relatively simple bacterial cells
can possess emergent properties due to purely physical interactions.
Therefore, accurate physico-genetic models of cell growth will be
essential for the design and understanding of genetically programmed
multicellular systems