This Week in PNAS
PHYSICS
Glass transition in space-time
Mauro Merolle et al. describe the statistical mechanics of the glass transition, where a supercooled fluid falls out of equilibrium, in terms of the time evolution of the states of a system (trajectories) rather than of the states themselves. Often, statistical mechanical theory of phase transitions and critical phenomena have been used to describe the glass transition. Merolle et al. present detailed analysis and numerical simulations for simple kinetically constrained models, with a particular variable of interest being the dynamical actions of these models. The authors show that the probability distributions of the actions have tails of low action that are greater than expected for a homogeneous system. These tails directly reflect the spatial correlations in the dynamics (dynamic heterogeneity) and indicate a coexistence between space-time regions where motion is plentiful versus regions where motion is rare. Therefore, according to the authors, the glass transition is a disorder-order transition in space-time. — R.N.
“Space-time thermodynamics of the glass transition” by Mauro Merolle, Juan P. Garrahan, and David Chandler (see pages 10837-10840)
EVOLUTION
Biogeographic analysis of Mexican tropical dry forests
Judith Becerra has characterized the temporal and spatial changes of diversity in Bursera, a common woody plant in the tropical dry forests of Mexico. The tropical dry forest is one of the most extensive types of vegetation of Mexico, but little is known about its history because of its meager fossil record. Because Bursera is highly adapted to the conditions of the tropical dry forest, it may be a useful indicator for the state of its habitat. In previous work, Becerra created a time-calibrated DNA phylogeny for the genus Bursera. Becerra found that, between 20 and 30 million years ago, Bursera began a relatively rapid diversification, which suggests that conditions were also favorable for tropical dry forests. Furthermore, the oldest Bursera lineages diverged mostly in western Mexico, whereas the more recent lineages diverged in the south-central part of the country, suggesting that the tropical dry forest followed these directions of expansion. The timing generally corresponds to the mountain-building activity of the Sierra Madre Occidental and Neovolcanic axis in western Mexico, which likely contributed to the conditions that encouraged the establishment and persistence of the tropical dry forest. — R.N.
“Timing the origin and expansion of the Mexican tropical dry forest” by Judith X. Becerra (see pages 10919-10923)
MEDICAL SCIENCES
Aquaporin 7 deficiency associated with obesity
Toshiyuki Hibuse et al. show that deficiency of the aquaporin 7 (AQP7) channel, which acts as a gateway for the efficient release of glycerol from adipocytes, is associated with adult-onset obesity in mice. Wild-type and AQP7-knockout (KO) mice maintained similar body weights until 10 weeks of age, at which point body weight and fat mass began to increase in the KO mice, with no increases in food intake. Obesity was also diet-induced, as the KO mice exhibited more rapid weight gain, as well as impairment in insulin-mediated suppression of plasma glucose, when fed a high-fat/high-sucrose diet. At 10 weeks of age, the adipose tissue of both mice contained similar mRNA levels of numerous genes involved in fatty acid uptake and metabolism; however, glycerol kinase (Gyk) activity was significantly higher in the adipose tissue of KO mice. Using AQP7 knockdown adipocytes, the authors highlighted the series of events leading to AQP7-associated obesity: AQP7 deficiency enhances Gyk enzymatic activity through increased intracellular glycerol content, which results in increased fatty acid uptake and triglyceride synthesis in adipocytes. — N.Z.
“Aquaporin 7 deficiency is associated with development of obesity through activation of adipose glycerol kinase” by Toshiyuki Hibuse, Norikazu Maeda, Tohru Funahashi, Kaori Yamamoto, Azumi Nagasawa, Wataru Mizunoya, Ken Kishida, Kazuo Inoue, Hiroshi Kuriyama, Tadashi Nakamura, Tohru Fushiki, Shinji Kihara, and Iichiro Shimomura (see pages 10993-10998)
MEDICAL SCIENCES
Early wave of 1918 flu in New York
Donald Olson et al. report that an early wave of the influenza virus during the 1918/1919 flu pandemic was likely present in New York City as early as February 1918. The 1918/1919 influenza pandemic killed >600,000 individuals in the United States and >40 million worldwide, and its origins remain elusive. Limited eyewitness reports and outbreak cases suggest that the virus first emerged in the United States in the spring of 1918 from a rural region in the Midwest. Olson et al. compared monthly age-specific mortality data for New York between 1911 and 1921 and found a shift in the age distribution of epidemic deaths between February and April of 1918. Individuals aged 15-44 experienced sharply elevated death rates during this time. Such a shift in the burden of excess mortality to younger age groups has been a characteristic feature of all three major 20th century influenza pandemics. Although samples of viral RNA would be needed to confirm that these mortalities were related to the 1918/1919 pandemic, these epidemiological data suggest that New York experienced a precursor wave of influenza that heralded the global outbreak to come. — N.Z.
“Epidemiological evidence of an early wave of the 1918 influenza pandemic in New York City” by Donald R. Olson, Lone Simonsen, Paul J. Edelson, and Stephen S. Morse (see pages 11059-11063)
Figures and Tables
Trajectories of supercooled fluid in glass transition.
Distribution of tropical dry forest (green) in Mexico, with main axes of Sierra Madre Occidental (A) and Neovolcanic belt (B).
Schematic of mechanism of obesity in AQP7-knockout mice.
