Sucrose Utilization in Budding Yeast as a Model for the Origin of Undifferentiated Multicellularity

We use the budding yeast, Saccharomyces cerevisiae, to investigate one model for the initial emergence of multicellularity: the formation of multicellular aggregates as a result of incomplete cell separation. We combine simulations with experiments to show how the use of secreted public goods favors the formation of multicellular aggregates. Yeast cells can cooperate by secreting invertase, an enzyme that digests sucrose into monosaccharides, and many wild isolates are multicellular because cell walls remain attached to each other after the cells divide. We manipulate invertase secretion and cell attachment, and show that multicellular clumps have two advantages over single cells: they grow under conditions where single cells cannot and they compete better against cheaters, cells that do not make invertase. We propose that the prior use of public goods led to selection for the incomplete cell separation that first produced multicellularity.Molecular and Cellular Biolog

Nutrient Shielding in Clusters of Cells

Cellular nutrient consumption is influenced by both the nutrient uptake kinetics of an individual cell and the cells' spatial arrangement. Large cell clusters or colonies have inhibited growth at the cluster's center due to the shielding of nutrients by the cells closer to the surface. We develop an effective medium theory that predicts a thickness ℓ of the outer shell of cells in the cluster that receives enough nutrient to grow. The cells are treated as partially absorbing identical spherical nutrient sinks, and we identify a dimensionless parameter ν that characterizes the absorption strength of each cell. The parameter ν can vary over many orders of magnitude among different cell types, ranging from bacteria and yeast to human tissue. The thickness ℓ decreases with increasing ν, increasing cell volume fraction ϕ, and decreasing ambient nutrient concentration $ψ_∞$. The theoretical results are compared with numerical simulations and experiments. In the latter studies, colonies of budding yeast, Saccharomyces cerevisiae, are grown on glucose media and imaged under a confocal microscope. We measure the growth inside the colonies via a fluorescent protein reporter and compare the experimental and theoretical results for the thickness ℓ.Molecular and Cellular BiologyPhysic

Improved Use of a Public Good Selects for the Evolution of Undifferentiated Multicellularity

We do not know how or why multicellularity evolved. We used the budding yeast, Saccharomyces cerevisiae, to ask whether nutrients that must be digested extracellularly select for the evolution of undifferentiated multicellularity. Because yeast use invertase to hydrolyze sucrose extracellularly and import the resulting monosaccharides, single cells cannot grow at low cell and sucrose concentrations. Three engineered strategies overcame this problem: forming multicellular clumps, importing sucrose before hydrolysis, and increasing invertase expression. We evolved populations in low sucrose to ask which strategy they would adopt. Of 12 successful clones, 11 formed multicellular clumps through incomplete cell separation, 10 increased invertase expression, none imported sucrose, and 11 increased hexose transporter expression, a strategy we had not engineered. Identifying causal mutations revealed genes and pathways, which frequently contributed to the evolved phenotype. Our study shows that combining rational design with experimental evolution can help evaluate hypotheses about evolutionary strategies.Molecular and Cellular Biolog

Sucrose Utilization in Budding Yeast as a Model for the Origin of Undifferentiated Multicellularity

We use the budding yeast, Saccharomyces cerevisiae, to investigate one model for the initial emergence of multicellularity: the formation of multicellular aggregates as a result of incomplete cell separation. We combine simulations with experiments to show how the use of secreted public goods favors the formation of multicellular aggregates. Yeast cells can cooperate by secreting invertase, an enzyme that digests sucrose into monosaccharides, and many wild isolates are multicellular because cell walls remain attached to each other after the cells divide. We manipulate invertase secretion and cell attachment, and show that multicellular clumps have two advantages over single cells: they grow under conditions where single cells cannot and they compete better against cheaters, cells that do not make invertase. We propose that the prior use of public goods led to selection for the incomplete cell separation that first produced multicellularity

Thin PDMS Films Using Long Spin Times or Tert-Butyl Alcohol as a Solvent

Thin polydimethylsiloxane (PDMS) films are frequently used in “lab on a chip” devices as flexible membranes. The common solvent used to dilute the PDMS for thin films is hexane, but hexane can swell the underlying PDMS substrate. A better solvent would be one that dissolves uncured PDMS but doesn't swell the underlying substrate. Here, we present protocols and spin curves for two alternatives to hexane dilution: longer spin times and dilution in tert-butyl alcohol. The thickness of the PDMS membranes under different spin speeds, spin times, and PDMS concentrations was measured using an optical profilometer. The use of tert-butyl alcohol to spin thin PDMS films does not swell the underlying PDMS substrate, and we have used these films to construct multilayer PDMS devices

The effects of expressive writing before or after punch biopsy on wound healing

Objective: Recent studies have shown that written emotional disclosure (expressive writing) performed in the two weeks prior to wounding improves healing of punch biopsy wounds. In many clinical settings, it would be more practical for patients to perform this intervention after wounding. The aim of this study was to investigate whether expressive writing could speed the healing of punch biopsy wounds if writing was performed after wounds were made. Methods: One hundred and twenty-two healthy participants aged between 18 and 55 years were randomly allocated to one of four groups in a 2 (intervention) by 2 (timing) design. Participants performed either expressive writing or neutral writing, either before or after receiving a 4 mm punch biopsy wound. Wounds were photographed on day 10 (primary endpoint) and day 14 after the biopsy to measure epithelisation. Participants also completed questionnaires on stress and affect two weeks prior to the biopsy, on the day of biopsy and two weeks after biopsy. Results: There was a significant difference in healing at day 10 between groups, χ2(3, N = 97) = 8.84, p = 0.032. A significantly greater proportion of participants who performed expressive writing before the biopsy had fully reepithelialised wounds on day 10 compared to participants who performed neutral writing either before or after wounding, with no other significant differences between groups. Amongst people who wrote expressively after wounding, those who finished writing over the first 6 days were significantly more likely to be healed at 14 days than those who finished writing later. There were significant differences in positive and negative affect over the healing period between the pre and post expressive writing groups. Conclusions: Expressive writing can improve healing if it is performed prior to wounding. Performing expressive writing after wounding may be able to improve healing depending on the timing of writing and wound assessment. Expressive writing causes affect to worsen followed by subsequent improvement and it is important to consider this in the timing of intervention delivery. Further research with patient groups is required to determine the clinical relevance of these findings

Estimating Plasma Glucose from Interstitial Glucose: The Issue of Calibration Algorithms in Commercial Continuous Glucose Monitoring Devices

Evaluation of metabolic control of diabetic people has been classically performed measuring glucose concentrations in blood samples. Due to the potential improvement it offers in diabetes care, continuous glucose monitoring (CGM) in the subcutaneous tissue is gaining popularity among both patients and physicians. However, devices for CGM measure glucose concentration in compartments other than blood, usually the interstitial space. This means that CGM need calibration against blood glucose values, and the accuracy of the estimation of blood glucose will also depend on the calibration algorithm. The complexity of the relationship between glucose dynamics in blood and the interstitial space, contrasts with the simplistic approach of calibration algorithms currently implemented in commercial CGM devices, translating in suboptimal accuracy. The present review will analyze the issue of calibration algorithms for CGM, focusing exclusively on the commercially available glucose sensors

Biomimetic Electrospun Coatings Increase the In Vivo Sensitivity of Implantable Glucose Biosensors

In vivo tissue responses and functional efficacy of electrospun membranes based on polyurethane (PU) and gelatin (GE) as biomimetic coatings for implantable glucose biosensors was investigated in a rat subcutaneous implantation model. Three electrospun membranes with optimized fibre diameters, pore sizes and permeability, both single PU and co-axial PU-GE fibres and a solvent cast PU film were implanted in rats to evaluate tissue responses. For functional efficacy testing, four sensor variants coated with the above mentioned electrospun membranes as mass-transport limiting and outermost biomimetic coatings were implanted in rats. The electrospun PU membranes had micron sized pores that were not permeable to host cells when implanted in the body. However, PU-GE coaxial fibre membranes, having similar sized pores, were infiltrated with fibroblasts that deposited collagen in the membrane’s pores. Such tissue response prevented the formation of dense fibrous capsule around the sensor coated with the PU-GE co-axial fibre membranes, which helped improve the in vivo sensitivity for at least 3 weeks compared to the traditional sensors in rat subcutaneous tissue. Furthermore, the better in vitro sensor’s sensitivity due to electrospun PU as the mass-transport limiting membrane translated to better in vivo sensitivity. Thus, this study showed that electrospun membranes can play an important role in realising long in vivo sensing lifetime of implantable glucose biosensors.This work was financially supported by the Royal Society, UK (Research Grant, Ref. No.: RG100129), and the National Institute of Health, USA (NIH/NIBIB, grant R01EB001640). Ning Wang acknowledges the studentship for her Ph.D. from Brunel Institute for Bioengineering, Brunel University. The authors also acknowledge the support and the guidance from Prof. Francis Moussy for this study

When increasing population density can promote the evolution of metabolic cooperation.

This is the author accepted manuscript. The final version is available from Nature Publishing Group via the DOI in this record.Microbial cooperation drives ecological and epidemiological processes and is affected by the ecology and demography of populations. Population density influences the selection for cooperation, with spatial structure and the type of social dilemma, namely public-goods production or self-restraint, shaping the outcome. While existing theories predict that in spatially structured environments increasing population density can select either for or against cooperation, experimental studies with both public-goods production and self-restraint systems have only ever shown that increasing population density favours cheats. We suggest that the disparity between theory and empirical studies results from experimental procedures not capturing environmental conditions predicted by existing theories to influence the outcome. Our study resolves this issue and provides the first experimental evidence that high population density can favour cooperation in spatially structured environments for both self-restraint and public-goods production systems. Moreover, using a multi-trait mathematical model supported by laboratory experiments we extend this result to systems where the self-restraint and public-goods social dilemmas interact. We thus provide a systematic understanding of how the strength of interaction between the two social dilemmas and the degree of spatial structure within an environment affect selection for cooperation. These findings help to close the current gap between theory and experiments.RJL and IG: European Research Council No. 647292 MathModExp. BJP: Engineering and Physical Sciences Research Council Doctoral training grant studentship