Mixl1 is required for axial mesendoderm morphogenesis and patterning in the murine embryo

In Xenopus, the Mix/Bix family of homeobox genes has been implicated in mesendoderm development. Mixl1 is the only known murine member of this family. To examine the role of Mixl1 in murine embryogenesis, we used gene targeting to create mice bearing a null mutation of Mixl1. Homozygous Mixl1 mutant embryos can be distinguished from their littermates by a marked thickening of the primitive streak. By the early somite stage, embryonic development is arrested, with the formation of abnormal head folds, foreshortened body axis, absence of heart tube and gut, deficient paraxial mesoderm, and an enlarged midline tissue mass that replaces the notochord. Development of extra-embryonic structures is generally normal except that the allantois is often disproportionately large for the size of the mutant embryo. In chimeras, Mixl1(-/-) mutant cells can contribute to all embryonic structures, with the exception of the hindgut, suggesting that Mixl1 activity is most crucial for endodermal differentiation. Mixl1 is therefore required for the morphogenesis of axial mesoderm, the heart and the gut during embryogenesis.     

RNA-binding proteins in human genetic disease

RNA-binding proteins (RBPs) are key components in RNA metabolism, regulating the temporal, spatial and functional dynamics of RNAs. Altering the expression of RBPs has profound implications for cellular physiology, affecting RNA processes from pre-mRNA splicing to protein translation. Recent genetic and proteomic data and evidence from animal models reveal that RBPs are involved in many human diseases ranging from neurologic disorders to cancer. Here we review the emerging evidence showing the involvement of RBPs in many disease networks and conclude that defects in RNA metabolism caused by aberrations in RBPs might underlie a broader spectrum of complex human disorders.     

Pax3( GFP ) , a new reporter for the melanocyte lineage, highlights novel aspects of PAX3 expression in the skin

The paired box gene 3 (Pax3) is expressed during pigment cell development. We tested whether the targeted allele Pax3(GFP) can be used as a reporter gene for pigment cells in the mouse. We found that enhanced green fluorescent protein (GFP) can be seen readily in every melanoblast and melanocyte in the epidermis and hair follicles of Pax3(GFP/+) heterozygotes. The GFP was detected at all differentiation stages, including melanocyte stem cells. In the dermis, Schwann cells and nestin-positive cells of the piloneural collars resembling the nestin-positive hair follicle multipotent stem cells exhibited a weaker GFP signal. Pigment cells could be purified by fluorescent activated cell sorting and grown in vitro without feeder cells, giving pure cultures of melanocytes. The Schwann cells and nestin-positive cells of the piloneural collars were FACS-isolated based on their weak expression of GFP. Thus Pax3(GFP) can discriminate distinct populations of cells in the skin.     

Borrelia burgdorferi Promotes the Establishment of Babesia microti in the Northeastern United States

Babesia microti and Borrelia burgdorferi, the respective causative agents of human babesiosis and Lyme disease, are maintained in their enzootic cycles by the blacklegged tick (Ixodes scapularis) and use the white-footed mouse (Peromyscus leucopus) as primary reservoir host. The geographic range of both pathogens has expanded in the United States, but the spread of babesiosis has lagged behind that of Lyme disease. Several studies have estimated the basic reproduction number (R ₀) for B. microti to be below the threshold for persistence (<1), a finding that is inconsistent with the persistence and geographic expansion of this pathogen. We tested the hypothesis that host coinfection with B. burgdorferi increases the likelihood of B. microti transmission and establishment in new areas. We fed I. scapularis larva on P. leucopus mice that had been infected in the laboratory with B. microti and/or B. burgdorferi. We observed that coinfection in mice increases the frequency of B. microti infected ticks. To identify the ecological variables that would increase the probability of B. microti establishment in the field, we integrated our laboratory data with field data on tick burden and feeding activity in an R ₀ model. Our model predicts that high prevalence of B. burgdorferi infected mice lowers the ecological threshold for B. microti establishment, especially at sites where larval burden on P. leucopus is lower and where larvae feed simultaneously or soon after nymphs infect mice, when most of the transmission enhancement due to coinfection occurs. Our studies suggest that B. burgdorferi contributes to the emergence and expansion of B. microti and provides a model to predict the ecological factors that are sufficient for emergence of B. microti in the wild.     

Quantitative determination of free cholesterol and cholesteryl esters in skin biopsies

A new technique is described for the determination of cholesterol in skin biopsies which is sensitive, practical and reproducible. The determination of cutaneous cholesterol is of clinical interest, because of its correlation with the degree of atherosclerosis. There was no correlation found between the blood total cholesterol and total triglycerides, and cholesterol and triglycerides determined in the skin biopsies. Determinations were carried out on several normal subjects, 7 long-distance runners, 34 hyperlipidemic patients. There was a significant increase with age of total skin cholesterol in the normal subjects, the values obtained with the long-distance runners had a tendency to be somewhat lower. All the patients investigated had higher cholesterol values than the normal controls or the sportsmen. This technique can be used therefore as a diagnostic tool to detect pathologies of skin lipids, or of tissue lipid metabolism, as for example in normolipidemic patients presenting corneal arcus or xanthelasmas.     

A method for genetic modification of human embryonic stem cells using electroporation

The ability to genetically modify human embryonic stem cells (HESCs) will be critical for their widespread use as a tool for understanding fundamental aspects of human biology and pathology and for their development as a platform for pharmaceutical discovery. Here, we describe a method for the genetic modification of HESCs using electroporation, the preferred method for introduction of DNA into cells in which the desired outcome is gene targeting. This report provides methods for cell amplification, electroporation, colony selection and screening. The protocol we describe has been tested on four different HESC lines, and takes approximately 4 weeks from electroporation to PCR screening of G418-resistant clones.     

100 years of psychology of concepts: the theoretical notion of concept and its operationalization

The operationalization of scientific notions is instrumental in enabling experimental evidence to bear on scientific propositions. Conceptual change should thus translate into operationalization change. This article describes some important experimental works in the psychology of concepts since the beginning of the twentieth century. It is argued that since the early days of this field, psychologists' theoretical understanding of concepts has been modified several times. However, in all cases but one, these theoretical changes did not translate into changes in the operationalization of the notion of concept learning.     

Salmonella-induced enteritis: molecular pathogenesis and therapeutic implications

Salmonella-induced enteritis is a gastrointestinal disease that causes major economic and welfare problems throughout the world. Although the infection is generally self-limiting, subgroups of the population such as immunocompromised individuals, the young and the elderly are susceptible to developing more severe systemic infections. The emergence of widespread antibiotic resistance and the lack of a suitable vaccine against enteritis-causing Salmonella have led to a search for alternative therapeutic strategies. This review focuses on how Salmonella induces enteritis at the molecular level in terms of bacterial factors, such as the type III secretion systems used to inject a subset of bacterial proteins into host cells, and host factors, such as Toll-like receptors and cytokines. The type III secreted bacterial proteins elicit a variety of responses in host cells that contribute to enteritis. Cytokines form part of the host defence mechanism, but in combination with bacterial factors can contribute to Salmonella-induced enteritis. We also discuss animal and cell culture models currently used to study Salmonella-induced enteritis, and how understanding the mechanisms of the disease has impacted on the development of Salmonella therapeutics.