Origin and segregation of cranial placodes in Xenopus laevis

Cranial placodes are local thickenings of the vertebrate head ectoderm that contribute to the paired sense organs (olfactory epithelium, lens, inner ear, lateral line), cranial ganglia and the adenohypophysis. Here we use tissue grafting and dye injections to generated fate maps of the dorsal cranial part of the non-neural ectoderm for Xenopus embryos between neural plate and early tailbud stages. We show that all placodes arise from a crescent-shaped area located around the anterior neural plate, the pre-placodal ectoderm. In agreement with proposed roles of Six1 and Pax genes in the specification of a panplacodal primordium and different placodal areas, respectively, we show that Six1 is expressed uniformly throughout most of the pre-placodal ectoderm, while Pax6, Pax3, Pax8 and Pax2 each are confined to specific subregions encompassing the precursors of different subsets of placodes. However, the precursors of the vagal epibranchial and posterior lateral line placodes, which arise from the posteriormost pre-placodal ectoderm, upregulate Six1 and Pax8/Pax2 only at tailbud stages. Whereas our fate map suggests that regions of origin for different placodes overlap extensively with each other and with other ectodermal fates at neural plate stages, analysis of co-labeled placodes reveals that the actual degree of overlap is much smaller. Time lapse imaging of the pre-placodal ectoderm at single cell resolution demonstrates that no directed, large-scale cell rearrangements occur, when the pre-placodal region segregates into distinct placodes at subsequent stages. Our results indicate that individuation of placodes from the pre-placodal ectoderm does not involve large-scale cell sorting in Xenopus.     

Allostery mediates ligand binding to WWOX tumor suppressor via a conformational switch

While being devoid of the ability to recognize ligands itself, the WW2 domain is believed to aid ligand binding to the WW1 domain in the context of a WW1–WW2 tandem module of WW domain‐containing oxidoreductase (WWOX) tumor suppressor. In an effort to test the generality of this hypothesis, we have undertaken here a detailed biophysical analysis of the binding of WW domains of WWOX alone and in the context of the WW1–WW2 tandem module to an array of putative proline‐proline‐x–tyrosine (PPXY) ligands. Our data show that while the WW1 domain of WWOX binds to all ligands in a physiologically relevant manner, the WW2 domain does not. Moreover, ligand binding to the WW1 domain in the context of the WW1–WW2 tandem module is two‐to‐three‐fold stronger than when treated alone. We also provide evidence that the WW domains within the WW1–WW2 tandem module physically associate so as to adopt a fixed spatial orientation relative to each other. Of particular note is the observation that the physical association of the WW2 domain with WW1 blocks access to ligands. Consequently, ligand binding to the WW1 domain not only results in the displacement of the WW2 lid but also disrupts the physical association of WW domains in the liganded conformation. Taken together, our study underscores a key role of allosteric communication in the ability of the WW2 orphan domain to chaperone physiological action of the WW1 domain within the context of the WW1–WW2 tandem module of WWOX. Copyright © 2015 John Wiley & Sons, Ltd.     

Molecular cytogenetic analysis and resequencing of contactin associated protein-like 2 in autism spectrum disorders

Autism spectrum disorders (ASD) are a group of related neurodevelopmental syndromes with complex genetic etiology. We identified a de novo chromosome 7q inversion disrupting Autism susceptibility candidate 2 (AUTS2) and Contactin Associated Protein-Like 2 (CNTNAP2) in a child with cognitive and social delay. We focused our initial analysis on CNTNAP2 based on our demonstration of disruption of Contactin 4 (CNTN4) in a patient with ASD; the recent finding of rare homozygous mutations in CNTNAP2 leading to intractable seizures and autism; and in situ and biochemical analyses reported herein that confirm expression in relevant brain regions and demonstrate the presence of CNTNAP2 in the synaptic plasma membrane fraction of rat forebrain lysates. We comprehensively resequenced CNTNAP2 in 635 patients and 942 controls. Among patients, we identified a total of 27 nonsynonymous changes; 13 were rare and unique to patients and 8 of these were predicted to be deleterious by bioinformatic approaches and/or altered residues conserved across all species. One variant at a highly conserved position, I869T, was inherited by four affected children in three unrelated families, but was not found in 4010 control chromosomes (p = 0.014). Overall, this resequencing data demonstrated a modest nonsignificant increase in the burden of rare variants in cases versus controls. Nonetheless, when viewed in light of two independent studies published in this issue of AJHG showing a relationship between ASD and common CNTNAP2 alleles, the cytogenetic and mutation screening data suggest that rare variants may also contribute to the pathophysiology of ASD, but place limits on the magnitude of this contribution.     

Functional Modeling and Phylogenetic Distribution of Putative Cylindrospermopsin Biosynthesis Enzymes

The alkaloid cylindrospermopsin is the most recently discovered cyanotoxin and has caused epidemic outbreaks of human poisoning. Cylindrospermopsin producing cyanobacteria have in recent times appeared in countries all over the world where they had not been observed previously and, thus, represent a global public health concern. Three putative cylindrospermopsin biosynthesis genes, encoding an amidinotransferase (aoaA), a nonribosomal peptide synthetase (aoaB), and a polyketide synthase (aoaC), have been described. Most cyanotoxins are the product of nonribosomal peptide and polyketide synthesis, but the involvement of an amidinotransferase is novel. In the present study, functional modeling was carried out to gain insight into the mechanism of precursor recruitment in cylindrospermopsin biosynthesis. In addition, the molecular phylogenies of putative cylindrospermopsin biosynthesis genes and producer organisms were determined. The model indicated that AoaA may catalyze the formation of guanidino acetate from glycine and arginine. The catalytic site of the AoaB adenylation domain provided two aspartate residues, instead of the usual one, which may be involved in the binding of the guanidino moiety of guanidino acetate. Molecular phylogenetic analysis grouped cylindrospermopsin producing cyanobacteria into two divergent groups. Although the phylogeny of the cylindrospermopsin biosynthesis genes followed that of the producer organisms, they were less divergent, which may indicate the recent horizontal transfer of these genes. [Reviewing Editor: Dr. Martin Kreitman]     

Rescue of an MMTV transgene by co-integration reveals novel locus control properties of the ovine β-lactoglobulin gene that confer locus commitment to heterogeneous tissues

In an attempt to enhance the frequency and level of expression of a poor-performing MMTV-driven transgene, we co-integrated this construct with the ovine -lactoglobulin (BLG) gene in transgenic mice. Seven lines of transgenic mice possessing co-integrated BLG and MMTV-RZ5 transgenes were compared with 12 lines of mice that possessed only the MMTV-RZ5 construct. Co-integration enhanced the frequency of expression in the mammary gland from two out of 12 lines for the MMTV-RZ5 transgene alone, to five out of seven when co-integrated with BLG. Surprisingly, co-integration also resulted in co-expression of the two transgenes in the salivary gland, lung and spleen in addition to the mammary gland. Furthermore, both transgenes were expressed in virgin animals, and throughout pregnancy and lactation, suggesting that the developmental regulation of the locus follows that of the MMTV-promoter. These findings represent a novel locus control property of the ovine BLG gene that confer s commitment of the locus to the mammary gland, but also to a range of heterogeneous tissues possibly defined by the second promoter at the locus     

A natural experiment identifies an impending ecological trap for a neotropical amphibian in response to extreme weather events

Extreme weather events are predicted to increase as a result of climate change, yet amphibian responses to extreme disturbance events remain understudied, especially in the Neotropics. Recently, an unprecedented windstorm within a protected Costa Rican rainforest opened large light gaps in sites where we have studied behavioral responses of diurnal strawberry poison frogs (Oophaga pumilio) to ultraviolet radiation for nearly two decades. Previous studies demonstrate that O. pumilio selects and defends perches where ultraviolet radiation (UV‐B) is relatively low, likely because of the lethal and sublethal effects of UV‐B. In this natural experiment, we quantified disturbance to O. pumilio habitat, surveyed for the presence of O. pumilio in both high‐disturbance and low‐disturbance areas of the forest, and assessed UV‐B levels and perch selection behavior in both disturbance levels. Fewer frogs were detected in high‐disturbance habitat than in low‐disturbance habitat. In general, frogs were found vocalizing at perches in both disturbance levels, and in both cases, in significantly lower UV‐B levels relative to ambient adjacent surroundings. However, frogs at perches in high‐disturbance areas were exposed to UV‐B levels nearly 10 times greater than males at perches in low‐disturbance areas. Thus, behavioral avoidance of UV‐B may not reduce the risks associated with elevated exposure under these novel conditions, and similarly, if future climate and human‐driven land‐use change lead to sustained analogous environments.     

Root to shoot ratio of crops as influenced by CO2

Crops of tomorrow are likely to grow under higher levels of atmospheric CO₂. Fundamental crop growth processes will be affected and chief among these is carbon allocation. The root to shoot ratio (R:S, defined as dry weight of root biomass divided by dry weight of shoot biomass) depends upon the partitioning of photosynthate which may be influenced by environmental stimuli. Exposure of plant canopies to high CO₂ concentration often stimulates the growth of both shoot and root, but the question remains whether elevated atmospheric CO₂ concentration will affect roots and shoots of crop plants proportionally. Since elevated CO₂ can induce changes in plant structure and function, there may be differences in allocation between root and shoot, at least under some conditions. The effect of elevated atmospheric CO₂ on carbon allocation has yet to be fully elucidated, especially in the context of changing resource availability. Herein we review root to shoot allocation as affected by increased concentrations of atmospheric CO₂ and provide recommendations for further research. Review of the available literature shows substantial variation in R:S response for crop plants. In many cases (59.5%) R:S increased, in a very few (3.0%) remained unchanged, and in others (37.5%) decreased. The explanation for these differences probably resides in crop type, resource supply, and other experimental factors. Efforts to understand allocation under CO₂ enrichment will add substantially to the global change response data base.Abbreviations R:S root to shoot ratio, dry weight basis