γ‐Aminobutyric acid (GABA) administration has been shown to increase β‐cell mass, leading to a reversal of type 1 diabetes in mice. Whether GABA has any effect on β cells of healthy and prediabetic/glucose‐intolerant obese mice remains unknown. In the present study, we show that oral GABA administration (ad libitum) to mice indeed increased pancreatic β‐cell mass, which led to a modest enhancement in insulin secretion and glucose tolerance. However, GABA treatment did not further increase insulin‐positive islet area in high fat diet‐fed mice and was unable to prevent or reverse glucose intolerance and insulin resistance. Mechanistically, whether in vivo or in vitro, GABA treatment increased β‐cell proliferation. In vitro, the effect was shown to be mediated via the GABAA receptor. Single‐cell RNA sequencing analysis revealed that GABA preferentially up‐regulated pathways linked to β‐cell proliferation and simultaneously down‐regulated those networks required for other processes, including insulin biosynthesis and metabolism. Interestingly, single‐cell differential expression analysis revealed GABA treatment gave rise to a distinct subpopulation of β cells with a unique transcriptional signature, including urocortin 3 (ucn3), wnt4, and hepacam2. Taken together, this study provides new mechanistic insight into the proliferative nature of GABA but suggests that β‐cell compensation associated with prediabetes overlaps with, and negates, its proliferative effects.—Untereiner, A., Abdo, S., Bhattacharjee, A., Gohil, H., Pourasgari, F., Ibeh, N., Lai, M., Batchuluun, B., Wong, A., Khuu, N., Liu, Y., Al Rijjal, D., Winegarden, N., Virtanen, C., Orser, B. A., Cabrera, O., Varga, G., Rocheleau, J., Dai, F. F., Wheeler, M. B. GABA promotes β‐cell proliferation, but does not overcome impaired glucose homeostasis associated with diet‐induced obesity. FASEB J. 33, 3968–3984 (2019). http://www.fasebj.org

Throughout the last 3 decades, Ebola virus (EBOV) outbreaks have been confined to isolated areas within Central Africa; however, the 2014 variant reached unprecedented transmission and mortality rates. While the outbreak was still under way, it was reported that the variant leading up to this outbreak evolved faster than previous EBOV variants, but evidence for diversifying selection was undetermined. Here, we test this selection hypothesis and show that while previous EBOV outbreaks were preceded by bursts of diversification, evidence for site-specific diversifying selection during the emergence of the 2014 EBOV clade is weak. However, we show strong evidence supporting an interplay between selection and correlated evolution (epistasis), particularly in the mucin-like domain (MLD) of the EBOV glycoprotein. By reconstructing ancestral structures of the MLD, we further propose a structural mechanism explaining how the substitutions that accumulated between 1918 and 1969 distorted the MLD, while more recent epistatic substitutions restored part of the structure, with the most recent substitution being adaptive. We suggest that it is this complex interplay between weak selection, epistasis, and structural constraints that has shaped the evolution of the 2014 EBOV variant. IMPORTANCEThe role that selection plays in the emergence of viral epidemics remains debated, particularly in the context of the 2014 EBOV outbreak. Most critically, should such evidence exist, it is generally unclear how this relates to function and increased virulence. Here, we show that the viral lineage leading up to the 2014 outbreak underwent a complex interplay between selection and correlated evolution (epistasis) in a protein region that is critical for immune evasion. We then reconstructed the three-dimensional structure of this domain and showed that the initial mutations in this lineage deformed the structure, while subsequent mutations restored part of the structure. Along this mutational path, the first and last mutations were adaptive, while the intervening ones were epistatic. Altogether, we provide a mechanistic model that explains how selection and epistasis acted on the structural constraints that materialized during the 2014 EBOV outbreak. The five viruses that constitute the genus Ebolavirus (Zaire ebolavirus [EBOV], Sudan ebolavirus, Bundibugyo ebolavirus, Reston ebolavirus, and Tai Forest ebolavirus) have been the cause of a major public health concern in sub-Saharan Africa for over 3 decades (1). Historically, outbreaks have been confined to isolated areas within Central Africa; however, the 2014 outbreak reached an unprecedented level, making this the largest outbreak since the discovery of the virus in 1976 (2).The primary analysis of EBOV isolates conducted by Gire and colleagues found a large number of nonsynonymous mutations between the 2014 EBOV sequences and all previously published EBOV sequences (3). In particular, 50 fixed nonsynonymous changes were observed, all of which were distinct to the 2014 EBOV vari...

Recent history has provided us with one pandemic (Influenza A/H1N1) and two severe viral outbreaks (Ebola and Zika). In all three cases, post-hoc analyses have given us deep insights into what triggered these outbreaks, their timing, evolutionary dynamics, and phylogeography, but the genomic characteristics of outbreak viruses are still unclear. To address this outstanding question, we searched for a common denominator between these recent outbreaks, positing that the genome of outbreak viruses is in an unstable evolutionary state, while that of non-outbreak viruses is stabilized by a network of correlated substitutions. Here, we show that during regular epidemics, viral genomes are indeed stabilized by a dense network of weakly correlated sites, and that these networks disappear during pandemics and outbreaks when rates of evolution increase transiently. Post-pandemic, these evolutionary networks are progressively re-established. We finally show that destabilization is not caused by substitutions targeting epitopes, but more likely by changes in the environment sensu lato. Our results prompt for a new interpretation of pandemics as being associated with evolutionary destabilized viruses.

Homogeneous populations of mature differentiated primary cell types can display variable responsiveness to extracellular stimuli, although little is known about the underlying mechanisms that govern such heterogeneity at the level of gene expression. In this article, we show that morphologically homogenous human endothelial cells exhibit heterogeneous expression of VCAM1 after TNF-a stimulation. Variability in VCAM1 expression was not due to stochasticity of intracellular signal transduction but rather to preexisting established heterogeneous states of promoter DNA methylation that were generationally conserved through mitosis. Variability in DNA methylation of the VCAM1 promoter resulted in graded RelA/p65 and RNA polymerase II binding that gave rise to a distribution of VCAM1 transcription in the population after TNF-a stimulation. Microarray analysis and single-cell RNA sequencing revealed that a number of cytokine-inducible genes shared this heterogeneous response pattern. These results show that heritable epigenetic heterogeneity is fundamental in inflammatory signaling and highlight VCAM1 as a metastable epiallele.