The Selective Advantage of the lac Operon for Escherichia coli Is Conditional on Diet and Microbiota Composition

The lac operon is one of the best known gene regulatory circuits and constitutes a landmark example of how bacteria tune their metabolism to nutritional conditions. It is nearly ubiquitous in Escherichia coli strains justifying the use of its phenotype, the ability to consume lactose, for species identification. Lactose is the primary sugar found in milk, which is abundant in mammals during the first weeks of life. However, lactose is virtually non-existent after the weaning period, with humans being an exception as many consume dairy products throughout their lives. The absence of lactose during adulthood in most mammals and the rarity of lactose in the environment, means that the selective pressure for maintaining the lac operon could be weak for long periods of time. Despite the ability to metabolize lactose being a hallmark of E. coli’s success when colonizing its primary habitat, the mammalian intestine, the selective value of this trait remains unknown in this ecosystem during adulthood. Here we determine the competitive advantage conferred by the lac operon to a commensal strain of E. coli when colonizing the mouse gut. We find that its benefit, which can be as high as 11%, is contingent on the presence of lactose in the diet and on the presence of other microbiota members in the gut, but the operon is never deleterious. These results help explaining the pervasiveness of the lac operon in E. coli, but also its polymorphism, as lac-negative E. coli strains albeit rare can naturally occur in the gut.

Variation, Variegation and Heritable Gene Repression in S. cerevisiae

Phenotypic heterogeneity provides growth advantages for a population upon changes of the environment. In S. cerevisiae, such heterogeneity has been observed as “on/off” states in the expression of individual genes in individual cells. These variations can persist for a limited or extended number of mitotic divisions. Such traits are known to be mediated by heritable chromatin structures, by the mitotic transmission of transcription factors involved in gene regulatory circuits or by the cytoplasmic partition of prions or other unstructured proteins. The significance of such epigenetic diversity is obvious, however, we have limited insight into the mechanisms that generate it. In this review, we summarize the current knowledge of epigenetically maintained heterogeneity of gene expression and point out similarities and converging points between different mechanisms. We discuss how the sharing of limiting repression or activation factors can contribute to cell-to-cell variations in gene expression and to the coordination between short- and long- term epigenetic strategies. Finally, we discuss the implications of such variations and strategies in adaptation and aging.

Synthetic multistability in mammalian cells

In multicellular organisms, gene regulatory circuits generate thousands of molecularly distinct, mitotically heritable states, through the property of multistability. Designing synthetic multistable circuits would provide insight into natural cell fate control circuit architectures and allow engineering of multicellular programs that require interactions among cells in distinct states. Here we introduce MultiFate, a naturally-inspired, synthetic circuit that supports long-term, controllable, and expandable multistability in mammalian cells. MultiFate uses engineered zinc finger transcription factors that transcriptionally self-activate as homodimers and mutually inhibit one another through heterodimerization. Using model-based design, we engineered MultiFate circuits that generate up to seven states, each stable for at least 18 days. MultiFate permits controlled state-switching and modulation of state stability through external inputs, and can be easily expanded with additional transcription factors. Together, these results provide a foundation for engineering multicellular behaviors in mammalian cells.

Robustness and evolvability in transcriptional regulation

The relationship between genotype and phenotype is central to our understanding of development, evolution, and disease. This relationship is known as the genotype- phenotype map. Gene regulatory circuits occupy a central position in this map, because they control when, where, and to what extent genes are expressed, and thus drive fundamental physiological, developmental, and behavioral processes in living organisms as different as bacteria and humans. Mutations that affect these gene expression patterns are often implicated in disease, so it is important that gene regulatory circuits are robust to mutation. Such mutations can also bring forth beneficial phenotypic variation that embodies or leads to evolutionary adaptations or innovations. Here we review recent theoretical and experimental work that sheds light on the robustness and evolvability of gene regulatory circuits.

Gene Circuit Explorer (GeneEx): an interactive web-app for visualizing, simulating and analyzing gene regulatory circuits

Summary GeneEx is an interactive web-app that uses an ODE-based mathematical modeling approach to simulate, visualize and analyze gene regulatory circuits (GRCs) for an explicit kinetic parameter set or for a large ensemble of random parameter sets. GeneEx offers users the freedom to modify many aspects of the simulation such as the parameter ranges, the levels of gene expression noise and the GRC network topology itself. This degree of flexibility allows users to explore a variety of hypotheses by providing insight into the number and stability of attractors for a given GRC. Moreover, users have the option to upload, and subsequently compare, experimental gene expression data to simulated data generated from the analysis of a built or uploaded custom circuit. Finally, GeneEx offers a curated database that contains circuit motifs and known biological GRCs to facilitate further inquiry into these. Overall, GeneEx enables users to investigate the effects of parameter variation, stochasticity and/or topological changes on gene expression for GRCs using a systems-biology approach. Availability and implementation GeneEx is available at https://geneex.jax.org. This web-app is released under the MIT license and is free and open to all users and there is no mandatory login requirement. Supplementary information Supplementary data are available at Bioinformatics online.

The miRNA-gene regulatory circuits in nSARS-CoV-2 and their potential correlations with pulmonary and thrombotic disorders

In view of the worldwide spread of the novel Severe Acute Respiratory Syndrome Coronavirus 2 (nSARS-CoV-2) infection pandemic situation, research to repurpose drugs, identify novel drug targets, vaccine candidates, diagnostic markers etc have created a new race to curb the disease. To uncover nSARS-CoV-2-related important biological features and understanding the molecular basis of this disease, network biology and miRNA-gene regulatory motif-based approach is used. 11 antiviral human-microRNAs (miRNAs) which can potentially target SARS-CoV-2 genes were collated; their direct miRNA interactors were identified and a comprehensive nSARS-CoV-2 responsive miRNA:Transcription Factor (TF):gene coregulatory network was built. 1385 miRNA:TF:gene tripartite, Feed-Forward Loops (FFLs) were identified from the network. The network topology was mapped into the biological space and the overrepresented pathways were identified. Four regulatory circuits: hsa-mir-9-5p-EP300-PLCB4, hsa-mir-324-3p-MYC-HLA-F, hsa-mir-1827-E2F1-CTSV and hsa-mir-1277-5p-SP1-CANX are identified. These miRNA-gene regulatory circuits are found to regulate signalling pathways like virus endocytosis, viral replication, inflammatory response, pulmonary vascularization, cell cycle control, virus spike protein stabilization, antigen presentation, etc. Some novel computational evidences for understanding nSARS-CoV-2 molecular mechanisms controlled by these regulatory circuits is put forth. The novel associations of miRNAs and genes identified with this infection are open for experimental validation. Further, these regulatory circuits also suggest potential correlations/similarity in the molecular mechanisms during nSARS-CoV-2 infection and pulmonary diseases and thromboembolic disorders. A detailed molecular snapshot of TGF-β signalling pathway as the common mechanism that could play an important role in controlling common pathophysiology i.e. systemic inflammation, increased pulmonary pressure, ground glass opacities, D-dimer overexpression is also put forth.

Dissipation in Non-Steady State Regulatory Circuits

In order to respond to environmental signals, cells often use small molecular circuits to transmit information about their surroundings. Recently, motivated by specific examples in signaling and gene regulation, a body of work has focused on the properties of circuits that function out of equilibrium and dissipate energy. We briefly review the probabilistic measures of information and dissipation and use simple models to discuss and illustrate trade-offs between information and dissipation in biological circuits. We find that circuits with non-steady state initial conditions can transmit more information at small readout delays than steady state circuits. The dissipative cost of this additional information proves marginal compared to the steady state dissipation. Feedback does not significantly increase the transmitted information for out of steady state circuits but does decrease dissipative costs. Lastly, we discuss the case of bursty gene regulatory circuits that, even in the fast switching limit, function out of equilibrium.

A model for the spatio-temporal design of gene regulatory circuits

The design of increasingly complex gene regulatory networks relies upon mathematical modelling to link the gap that goes from conceptualisation to implementation. An overarching challenge is to update modelling abstractions and assumptions as new mechanistic information arises. Although models of bacterial gene regulation are often based on the assumption that the role played by intracellular physical distances between genetic elements is negligible, it has been shown that bacteria are highly ordered organisms, compartmentalizing their vital functions in both time and space. Here, we analysed the dynamical properties of regulatory interactions by explicitly modelling spatial constraints. Key to the model is the combined search by a regulator for its target promoter via 1D sliding along the chromosome and 3D diffusion through the cytoplasm. Moreover, this search was coupled to gene expression dynamics, with special attention to transcription factor-promoter interplay. As a result, promoter activity within the model depends on its physical separation from the regulator source. Simulations showed that by modulating the distance between DNA components in the chromosome, output levels changed accordingly. Finally, previous experimental results with engineered bacteria in which this distance was minimized or enlarged were successfully reproduced by the model. This suggests that the spatial specification of the circuit alone can be exploited as a design parameter to select programmable output levels.

The concerted action of E2-2 and HEB is critical for early lymphoid specification

The apparition of adaptive immunity in Gnathostomata correlates with the expansion of the E-protein family to encompass E2-2, HEB and E2A. Within the family, E2-2 and HEB are more closely evolutionarily related but their concerted action in hematopoiesis remains to be explored. Here we show that the combined disruption of E2-2 and HEB results in failure to express the early lymphoid program in CLPs and a near complete block in B-cell development. In the thymus, ETPs were reduced and T-cell development perturbed, resulting in reduced CD4 T- and increased γδ T-cell numbers. In contrast, HSCs, erythro-myeloid progenitors and innate immune cells were unaffected showing that E2-2 and HEB are dispensable for the ancestral hematopoietic lineages. Taken together, this E-protein dependence suggests that the appearance of the full Gnathostomata E-protein repertoire was critical to reinforce the gene regulatory circuits that drove the emergence and expansion of the lineages constituting humoral immunity.