Biology subjects

Brent, R.

Publications and source records attributed to Brent, R..

9 recordsLinked to original sources

bioRxiv2026

Effects of dose-response relationships on cellular information transmission

Cell signaling systems can transmit information with high precision, despite relying on complex reaction networks and inherently noisy biochemical reactions. To better understand how they achieve this precision, this work focuses on how cells can tune their quantitative steady-state dose-response relationships to improve signaling fidelity. Using a combination of analytic and numerical approaches, we quantify the maximum information that can be transmitted in single-step and multi-step signaling systems under several biologically realistic noise distributions: Gaussian, gamma, constant coeffcient of variation, and binomial. We find that signaling systems transmit more information if their dose-response curves have low baselines, large dynamic ranges, and curve shapes that are well matched to the natural distribution of input signals. For multi-step systems, linear transfer functions typically improve signaling fidelity for all noise levels, consistent with previous work on "dose-response alignment". We also find that switchlike transfer functions transmit a maximum of one bit and are robust to low levels of noise, but fail rapidly with high levels of noise. Simulations of toy signaling systems corroborate these results. Together, these findings provide insight into some cellular design principles that have arisen through evolution and that are widely observed in nature.

systems biology↗

bioRxiv2024

Evolverator: An engineered in cellulo yeast system to drive rapid continuous evolution of proteins

Continuous in vivo evolution of non-essential proteins requires an engineered link between target protein and host cell fitness, which is difficult to establish and vulnerable to escape mutations. We systematically designed and tested Saccharomyces cerevisiae gene circuits guided by quantitative mathematical models that predicted circuit, organism, and population behavior. We identified four generalizable design principles for escape-resistant continuous evolution and built the Evolverator, a yeast platform that couples protein binding to proliferation rate while suppressing escape mutations. We used the Evolverator to improve human estrogen receptor ligand affinity and generate stronger yeast repressors from the bacterial LacI protein. The resulting variants revealed previously undescribed hER substitutions within and outside the binding pocket, as well as LacI mutations that enhance repression in yeast despite reduced activity in Escherichia coli. Our work establishes a generalizable, model-guided framework for building yeast strains to allow continuous evolution of diverse protein functions in vivo.

synthetic biology↗

bioRxiv2022

Autorepression-based conditional gene expression system in yeast for variation-suppressed control of protein dosage

Conditional control of gene expression allows an experimenter to investigate many aspects of a genes function. In the model organism Saccharomyces cerevisiae, a number of methods to control gene expression are widely practiced, including induction by metabolites, small molecules, and even light. However, all current methods suffer from at least one of a set of drawbacks, including need for specialized growth conditions, leaky expression, or the requirement of specialized equipment. Here we describe protocols using two transformations to construct strains that carry a new controller, in which all these drawbacks are overcome. In these strains, the expression of a controlled gene (gene of interest, or GOI) is repressed by the bacterial repressor TetR, and induced by anhydrotetracycline. TetR also regulates its own expression, creating an autorepression loop. This autorepression allows tight control of gene expression/ protein dosage with low cell to cell variation in expression. A second repressor, TetR-Tup1, prevents any leaky expression. We also present a protocol showing a particular workhorse application of such strains, to generate synchronized cell populations. We turn off the expression of the cell cycle regulator CDC20 completely, arresting the cell population, and then back on so that the synchronized cells resume cell cycle progression. This control system can be applied to any endogenous or exogenous gene for precise expression. Basic Protocol 1Generating a parent WTC846 strain. Basic Protocol 2Generating a WTC846 strain with controlled expression of the targeted gene Alternate Protocol 1CRISPR-mediated promoter replacement Basic Protocol 3Cell cycle synchronization/Arrest and Release using the WTC846-K3::CDC20 strain

genetics↗

bioRxiv2020

Individual yeast cells signal at different levels but each with good precision

Different isogenic cells have been shown to exhibit widely varying responses to the same extracellular signal. Based on the assumption that this variation arises from noise in the signaling pathways that cells use to transmit information from surface to nucleus, recent publications asserted that single cells cannot detect their surroundings accurately. Here, we analyze existing data on gene expression induced by the Saccharomyces cerevisiae pheromone response system, finding that individual cells signal consistently over time, implying that response variation arises primarily from stable cell-to-cell differences rather than signaling noise. Individual cells transmit at least 2.7 bits of information through the pheromone response system, enabling each cell to distinguish between at least 6 pheromone concentrations. In principle, cells can gain further precision by internally referencing these responses with measurements of constitutively expressed genes. Combination with prior results shows that only about 6% of total response variation arises from signaling pathway noise. One-sentence summarySingle yeast cells signal consistently over time, indicating that their signaling pathways transmit information precisely.

systems biology↗

bioRxiv2019

A precisely-titratable, variation-suppressed transcriptional controller to enable genetic discovery

Methods to express genes conditionally into phenotype remain central to biological experimentation and biotechnology. Current methods enable either on/off or imprecisely controlled graded gene expression. We developed a "well-tempered" controller, WTC846, for precisely adjustable, graded and growth condition independent conditional expression of genes in Saccharomyces cerevisiae. In WTC846 strains, the controlled genes are expressed from a strong, native promoter engineered to be repressed by the prokaryotic TetR protein and induced by tetracycline and analogues. A second instance of this promoter drives TetR itself. This autorepression loop exhibits low cell-to-cell variation in gene expression and allows precise adjustment of the steady state abundance of any protein with inducer. A second, constitutively expressed zeroing repressor abolishes basal expression in the absence of inducer. WTC846-controlled, stable (Cdc42, Tpi1) and unstable (Ipl1) proteins recapitulated known knockout and overexpression phenotypes. WTC846::CDC20 strains enabled inducer regulated cell cycle synchronization. WTC846 alleles of CDC28, TOR1, PBR1 and PMA1 exhibited expected gene dosage-dependent growth rates and morphological phenotypes, and WTC846::WHI5 strains exhibited inducer controlled differences in cell volume. WTC846 controlled genes comprise a new kind of "expression clamped" allele, for which variation in expression is minimized and gene dosage can be set by the experimenter across the range of cellular protein abundances. In yeast, we expect WTC846 alleles to find use in assessment of phenotypes now incompletely penetrant due to variable dosage of the causative protein, and in genome-wide epistasis screens. Implementation in higher cells should enable experiments now impossible due to cell-to-cell variation and imprecise control.

molecular biology↗

bioRxiv2018

In vivo measurements reveal a single 5’ intron is sufficient to increase protein expression level in C. elegans

Introns can increase gene expression levels using a variety of mechanisms collectively referred to as Intron Mediated Enhancement (IME). To date, the magnitude of IME has been quantified in human cell culture and plant models by comparing intronless reporter gene expression levels to those of intron-bearing reporter genes in vitro (mRNA, Western Blots, protein activity), using genome editing technologies that lacked full control of locus and copy number. Here, for the first time, we quantified IME in vivo, in terms of protein expression levels, using fluorescent reporter proteins expressed from a single, defined locus in Caenorhabditis elegans. To quantify the magnitude of IME, we developed a microfluidic chip-based workflow to mount and image individual animals, including software for operation and image processing. We used this workflow to systematically test the effects of position, number and sequence of introns on two different proteins, mCherry and mEGFP, driven by two different promoters, vit-2 and hsp-90. We found the three canonical synthetic introns commonly used in C. elegans transgenes increased mCherry protein concentration by approximately 50%. The naturally-occurring introns found in hsp-90 also increased mCherry expression level by about 50%. Furthermore, and consistent with prior results examining mRNA levels, protein activity or phenotypic rescue, we found that a single, natural or synthetic, 5 intron was sufficient for the full IME effect while a 3 intron was not. IME was also affected by protein coding sequence (50% for mCherry and 80% for mEGFP) but not strongly affected by promoter 46% for hsp-90 and 54% for the stronger vit-2. Our results show that IME of protein expression in C. elegans is affected by intron position and contextual coding sequence surrounding the introns, but not greatly by promoter strength. Our combined controlled transgenesis and microfluidic screening approach should facilitate screens for factors affecting IME and other intron-dependent processes.

genetics↗

bioRxiv2018

Differences in protein dosage underlie nongenetic differences in traits

Phenotypic expression of many traits varies among isogenic individuals in homogeneous environments. Intrinsic variation in the protein chaperone system affects a wide variety of traits in diverse biological systems. In C. elegans, expression of hsp-16.2 chaperone biomarkers predicts the penetrance of mutations and lifespan after heat shock. But the physiological mechanisms by which cells express different amounts of the biomarker were unknown. Here, we used an in vivo microscopy approach to dissect the mechanisms of cell-to-cell variation in hsp-16.2 biomarker expression, focusing on the intestines, which generate most signal. We found both intrinsic noise and signaling noise are low. The major axis of cell-to-cell variation in gene expression is composed of general differences in protein dosage. Thus, hsp-16.2 biomarkers reveal states of high or low effective dosages for many genes. It is possible that natural variation in protein dosage or chaperone activity may account for missing heritability of some traits.

systems biology↗

bioRxiv2016

Cell-to-cell variability in the yeast pheromone response: high throughput screen identifiesgenes with different effects on transmitted signal and response

Populations of isogenic cells often respond coherently to signals despite differences in protein abundance and cell state. Our previous work in the Saccharomyces cerevisiae pheromone response system (PRS) uncovered processes that reduced cell-to-cell variation in signal and response. To understand these and other processes that controlled variation, we generated a whole-genome collection of haploid strains with deletions in non-essential genes and used high-throughput flow cytometry to screen more than 1000. We identified 50 \"variation genes\" required for normal cell-to-cell variability in signal and response. Some genes affected only signal variability, signal strength, or system output, defining these quantities as separable \"axes\" of system behavior. Two genes affected cytoplasmic microtubule function.

systems biology↗

bioRxiv2016

Cell-to-cell variability in the yeast pheromone response: Cytoplasmic microtubule functionstabilizes signal generation and promotes accurate fate choice

In a companion paper, we carried out a high-throughput screen to identify genes that suppressed cell-to-cell variability in signaling in yeast. Two genes affected cytoplasmic microtubules that can connect the nucleus to a signaling site on the membrane. Here, we show that microtubule perturbations that affected polymerization and depolymerization, membrane attachment, and force generation increased variability. For some perturbations, \"outlier\" cells drove the increased variability. Bypass experiments that activated the PRS ectopically at downstream points indicated that microtubule-dependent processes might stabilize the membrane-recruited scaffold protein Ste5. The variability caused by microtubule perturbations required the MAP kinase Fus3. Microtubule perturbations hindered stable scaffold formation and decreased the accuracy of a polarity-dependent fate choice. Our experiments suggest that membrane-attached microtubules stabilize signaling by scaffold-bound Fus3, and are consistent with a model in which signaling irregularities from changes in microtubule function are amplified by cross-stimulatory feedbacks among PRS proteins. The fact that microtubule perturbations also cause aberrant fate and polarity decisions during embryonic development and cancer initiation suggests that similar variation-reducing processes might also operate in metazoans.

systems biology↗