This Week in PNAS
BIOPHYSICS
Single molecule flips between dim and bright states
Satoshi Habuchi et al. have characterized a reversible photoswitching system that uses individual molecules of Dronpa, a mutant of GFP cloned from the coral Pectiniidae. The authors showed that a single Dronpa molecule can flip between bright and dim states when exposed to light wavelengths of 405 and 488 nm, respectively. Dronpa can switch back and forth >100 times on a millisecond time scale. Previous reports have demonstrated photolevel switching with other GFP mutants, though many of these required minutes of illumination to trigger the switch, resulting in slow response times. Habuchi et al. suggest that the key to Dronpa's rapid switching behavior may be that minimal structural change is required to flip from dim to bright. The authors also note that, because the chromophore in Dronpa is buried deep within its protein structure, the switching mechanism is protected from environmental influences, thus possibly making it a robust optical tag. Dronpa could be particularly useful for optically tracking the diffusion or transport of molecules in live cells. The authors say such a molecular tag could enable the development of molecular-scale devices for use in optoelectronics and bioimaging. — B.T.
“Reversible single-molecule photoswitching in the GFP-like fluorescent protein Dronpa” by Satoshi Habuchi, Ryoko Ando, Peter Dedecker, Wendy Verheijen, Hideaki Mizuno, Atsushi Miyawaki, and Johan Hofkens (see pages 9511–9516)
IMMUNOLOGY
Evolution of vertebrate Toll-like receptors
According to Jared Roach et al., phylogenetic analysis shows that strong selective pressure for recognition of pathogen-associated molecular patterns (PAMPs) helps to maintain a largely unchanging repertoire of Toll-like receptors (TLRs) in vertebrates. The TLR multigene family encodes recognition receptors of the innate immune system. Mining publicly available databases, the researchers aligned TLR amino acid sequences from a diverse group of vertebrate species. Molecular distances were computed and molecular trees constructed to show that nearly all vertebrate TLRs fall into one of six major families. TLRs within a family recognized a specific class of PAMP. All families and all genes within each family were roughly equidistant from the center of the molecular tree. Suggesting that TLRs are evolving at similar rates and little coincidental evolution has occurred, Roach et al. hypothesize that this conservative evolution makes the receptors exceptional, because other multigene families of the immune system do not maintain such clear-cut orthologies within subfamilies. — F.A.
“The evolution of vertebrate Toll-like receptors” by Jared C. Roach, Gustavo Glusman, Lee Rowen, Amardeep Kaur, Maureen K. Purcell, Kelly D. Smith, Leroy E. Hood, and Alan Aderem (see pages 9577–9582)
NEUROSCIENCE
DJ-1 protects against cell death from oxidative stress
Eunsung Junn et al. observe that the multifunctional protein DJ-1 appears to protect against cell death induced by oxidative stress by quenching reactive oxygen species and sequestering the death protein Daxx in the nucleus. Previous research has identified a DJ-1 mutation in a subset of patients with familial Parkinson's disease. Junn et al. engineered a dopaminergic neuroblastoma cell line overexpressing DJ-1 and studied its properties. Wild-type DJ-1 protected the cells against hydrogen peroxide-induced oxidative stress up to 5-fold greater than control cells. By screening adult human brain cDNAs using wild-type DJ-1 as bait, the authors recovered Daxx, which activates apoptosis signal-regulating kinase 1 (ASK1) protein. Wild-type DJ-1 was shown to bind Daxx tighter than mutant DJ-1, and both DJ-1 and Daxx were found to localize to the nucleus. The authors cotransfected COS-7 cells with wild-type or mutant DJ-1, Daxx, and ASK1 and observed significant cytoprotection in cells with the wild-type protein. In addition, the expression of wild-type DJ-1 blocked the translocation of Daxx from the nucleus to the cytosol, inhibiting Daxx's ability to activate ASK1 and induce apoptosis. — F.A.
“Interaction of DJ-1 with Daxx inhibits apoptosis signal-regulating kinase 1 activity and cell death” by Eunsung Junn, Hiroyuki Taniguchi, Byeong Seon Jeong, Xin Zhao, Hidenori Ichijo, and M. Maral Mouradian (see pages 9691–9696)
POPULATION BIOLOGY
Stochastic model explores cancer drug resistance
Natalia Komarova and Dominik Wodarz have developed a mathematical model to explore the principles by which drug resistance emerges in cancer therapy. In recent years, cancer treatments have moved increasingly toward targeted therapy, which uses small molecule drugs to correct particular molecular abnormalities underlying a tumor's growth. Although targeted therapy has shown clinical promise, an obstacle to its success is the drug resistance attained by some tumors, especially in late-stage cancers. Komarova and Wodarz formulated a stochastic birth–death process to describe the behavior of cancerous cells before and after drug treatment, with measurable parameters such as turnover rate of tumor cells and the rate at which resistant mutant cells are generated. For all realistic cases, drug resistance was shown to develop predominantly before the start of treatment. According to the model, using a combination of drugs is unlikely to provide an advantage over single-drug therapy for cancers with a high turnover rate. However, applying the model to chronic myeloid leukemia, a combination of three targeted drugs with different specificities might overcome the problem of resistance. — R.N.
“Drug resistance in cancer: Principles of emergence and prevention” by Natalia L. Komarova and Dominik Wodarz (see pages 9714–9719)
SOCIAL SCIENCES
Analysis of interclass differences in education
Factors contributing to social class differentials in educational attainment have been extensively studied. Some researchers believe that, in addition to the primary effect of interclass differences in academic performance, there is also a secondary effect of interclass differences in educational choices made at given levels of performance. Robert Erikson et al. investigated the relative importance of these effects with cohort data from English and Welsh youth. The authors focused on three variables: social class background, level of academic performance, and choice in the transition to the next level of education (A-levels) at age 16. The authors used a counterfactual analysis, implemented by numerical integration and supported by an analytical explanation. Their model allows the choice characteristics of students of one social class to be combined with the performance distribution of students of another class to produce a counterfactual or potential outcome. In this way, primary and secondary effects can be separated, and their relative importance can be evaluated. The researchers found that student choice considered independently of performance accounts for at least 25% of overall class differences in the proportions of students continuing to A-levels. — R.N.
“On class differentials in educational attainment” by Robert Erikson, John H. Goldthorpe, Michelle Jackson, Meir Yaish, and D. R. Cox (see pages 9730–9733)
Figures and Tables
Bright–dim photoswitching by Dronpa.
Molecular tree of vertebrate TLR.
DJ-1 inhibits translocation of Daxx to cytosol.
Emergence/extinction of susceptible (green) and resistant (red) tumor cells before and after drug treatment.
