Cholesterol is an essential but intractable sludge-like substance for which mammalian cells must go to great lengths to synthesize, transport, and dispose of. The properties of the cholesterol molecule that render it an attractive component of cellular membranes, namely a rigid tertiary structure and exceptional hydrophobicity, also make it a metabolic bear to deal with. To overcome this mulish behavior, hundreds of proteins are dedicated to man-handling cholesterol into and out of the membrane, and dozens more regulate its expression. Disposal presents a particularly challenging problem as cholesterol is extremely hydrophobic. Sixteen or more hepatic enzymes catalyze a concerted series of reactions that hydroxylate, isomerize, oxidize, reduce, and twist the sterol into a hydrophilic conjugated bile acid that is readily excreted from the liver into the bile and thereafter from the organism.
the 24 March issue of Nature, researchers report that in the mustard plant Arabidopsis thaliana, gene inheritance can somehow skip generations: Plants sometimes end up with their grandparents' good copy of a gene instead of the mutant ones belonging to their parents.
Germ-free mice were maintained on polysaccharide-rich or simple-sugar diets and colonized for 10 days with an organism also found in human guts, Bacteroides thetaiotaomicron, followed by whole-genome transcriptional profiling of bacteria and mass spectrometry of cecal glycans. We found that these bacteria assembled on food particles and mucus, selectively induced outer-membrane polysaccharide-binding proteins and glycoside hydrolases, prioritized the consumption of liberated hexose sugars, and revealed a capacity to turn to host mucus glycans when polysaccharides were absent from the diet. This flexible foraging behavior should contribute to ecosystem stability and functional diversity.
The gut immune system has the challenge of responding to pathogens while remaining relatively unresponsive to food antigens and the commensal microflora. In the developed world, this ability appears to be breaking down, with chronic inflammatory diseases of the gut commonplace in the apparent absence of overt infections. In both mouse and man, mutations in genes that control innate immune recognition, adaptive immunity, and epithelial permeability are all associated with gut inflammation. This suggests that perturbing homeostasis between gut antigens and host immunity represents a critical determinant in the development of gut inflammation and allergy.
The distal human intestine represents an anaerobic bioreactor programmed with an enormous population of bacteria, dominated by relatively few divisions that are highly diverse at the strain/subspecies level. This microbiota and its collective genomes (microbiome) provide us with genetic and metabolic attributes we have not been required to evolve on our own, including the ability to harvest otherwise inaccessible nutrients. New studies are revealing how the gut microbiota has coevolved with us and how it manipulates and complements our biology in ways that are mutually beneficial. We are also starting to understand how certain keystone members of the microbiota operate to maintain the stability and functional adaptability of this microbial organ.
The function of an organ is dependent on its cellular constituents as well as on their assembly into a cohesive unit. The developing gut faces unique challenges as one of the longest and largest organs in the body and also because it is constantly interfacing with external factors through the diet. Its location deep within the body has until recently hampered investigation into its formation. The patterning of the gut along its longitudinal, dorsoventral, left-right, and radial axes is one of the fascinating issues that pertain to the development, function, and homeostasis of this understudied organ.
The extracellular environment is largely comprised of complex polysaccharides, which were historically considered inert materials that hydrated the cells and contributed to the structural scaffolds. Recent advances in development of sophisticated analytical techniques have brought about a dramatic transformation in understanding the numerous biological roles of these complex polysaccharides. Glycosaminoglycans (GAGs) are a class of these polysaccharides, which bind to a wide variety of proteins and signaling molecules in the cellular environment and modulate their activity, thus impinging on fundamental biological processes. Despite the importance of GAGs modulating biological functions, there are relatively few examples that demonstrate specificity of GAG-protein interactions, which in turn define the structure-function relationships of these polysaccharides. Focusing on heparin/heparan (HSGAGs) and chondroitin/dermatan sulfate (CSGAGs), this review provides structural insights into the oligosaccharide-protein interactions and discusses some key and challenging aspects of understanding GAG structure-function relationships.
In this issue of Chemistry & Biology, a strategy that combines large DNA fragment recombineering in Escherichia coli and heterologous expression in Pseudomonas putida is described. The work focuses on myxochromide S, a natural compound produced by Stigmatella aurantiaca.
Inhibitex’s core technology may not be initially obvious, but it represents a strong potential advance in the fight against severe bacterial and fungal infection. MSCRAMM proteins, or Microbial Surface Components Recognizing Adhesive Matrix Molecules, are at the heart of Alpharetta, GA-based Inhibitex’s anti-infective efforts and are the foundation of two products in clinical testing. “MSCRAMM technology involves the characterization and identification of surface proteins on pathogenic organisms that allow the adhesion of the organism to host tissues,” explains Joseph M. Patti, Ph.D., Vice President of Preclinical Development and Chief Scientific Officer at Inhibitex. “We generate antibodies against these MSCRAMM proteins to prevent or interfere with the adhesion process, the first step in initiation of the host infection.”
Cell-cell signaling is a central process in the formation of multicellular organisms. Notch (N) is the receptor of a conserved signaling pathway that regulates numerous developmental decisions, and the misregulation of N has been linked to various physiological and developmental disorders. The endocytosis of N and its ligands is a key mechanism by which N-mediated cell-cell signaling is developmentally regulated. We review here the recent findings that have highlighted the importance and complexity of this regulation.
Neural crest precursor cells appear in vertebrate embryos around mid-gastrulation and are maintained as multipotent cells until neural tube closure, after which they migrate and differentiate into the peripheral nervous system and other tissues. Light and colleagues now report that, in Xenopus, Id3 acts downstream of Myc to prevent neural crest precursor cells prematurely losing their multipotency (see p. 1831). Myc has been previously shown to prevent premature cell fate decisions in neural crest precursor cells. Light et al. show that the morpholino-mediated knockdown of Id3 – Id proteins are negative regulators of bHLH transcription factors – produces embryos in which CNS progenitors replace neural crest. Conversely, forced expression of Id3 maintains neural precursors in an undifferentiated state. Thus, Id3 helps to maintain neural crest stem cells until the appropriate time for them to respond to differentiation signals. Similar strategies may help to maintain other stem cell populations.
During the development of the retina, platelet-derived growth factor released by retinal neurons stimulates the proliferation of astrocytes, which release vascular endothelial growth factor (VEGF) to stimulate blood vessel growth. On p. 1855, West and co-workers describe how the developing retinal vessels provide feedback signals that trigger astrocyte differentiation, thus downregulating VEGF production and preventing runaway angiogenesis. In newborn mice raised in a high-oxygen atmosphere, retinal blood vessel development is blocked, resulting in retinal hypoxia. In this situation, report the researchers, retinal astrocytes continue to proliferate and fail to differentiate, indicating that retinal blood vessels usually limit their own formation by signalling to the astrocytes. Culturing astrocytes in a low-oxygen atmosphere reproduces these effects on astrocyte behaviour, suggesting that blood-borne oxygen might be the signal that indirectly prevents the over-elaboration of the vascular network. Similar feedback mechanisms may also govern vascularisation elsewhere in the nervous system.
The conserved Aristaless-related homeodomain protein ARX is essential for neuronal development in vertebrates. ARX mutations underlie multiple forms of human X-linked mental retardation and Arx null mice have decreased neural precursor proliferation and defects in GABAergic interneuron differentiation. To determine the precise role of ARX in neuronal development, Melkman and Sengupta are studying the C. elegans homologue of Arx, called alr-1 (see p. 1935). They report that, similar to its Drosophila orthologue, alr-1 acts in a pathway with the LIM1 orthologue lin-11 to specify a subset of chemosensory neurons. In addition, alr-1 is required for the differentiation of a GABAergic motoneuron subtype. These data suggest that, like other homeodomain proteins, some functions of ARX are conserved across species. Consequently, future studies of the simple nervous system of C. elegans may yield insights into the role of ARX in human neuronal development.
At first glance, the eyes of chicks and Drosophila have little in common. However, on p. 1895, Blanco and co-workers report that at least one of the genetic regulatory circuits involved in eye development has been largely conserved during evolution. In chick, lens-specific regulation of the 1-crystallin gene is achieved by the cooperative binding of the transcription factors PAX6 and SOX2 to the 30 bp DC5 fragment within the gene's enhancer. Using reporter genes, Blanco et al. show that the DC5 fragment is active in the Drosophila compound eye, specifically in the cone cells that secrete Crystallin. Other studies, including a loss-of-function analysis, indicate that the DC5 element in Drosophila is regulated by the cooperative binding of D-PAX2 and SOXN. Since PAX6 and PAX2 derive from the same ancestor, this indicates that while Pax6 took over Crystallin regulation in vertebrates during evolution, Pax2 retained this function in flies.
Asymmetric cell division generates cell diversity in all organisms. After asymmetric division, daughter cells acquire distinct cell fates by transcribing different sets of genes. Now, Yoda and colleagues report that LET-19 and DPY-22 – components of the transcriptional Mediator complex – are required for asymmetric division in C. elegans (see p. 1885). They identify mutations in let-19 and dpy-22 that disrupt the asymmetry of T-cell division and cause the symmetrical expression of tlp-1 in T-cell daughters; the normally asymmetric expression of this transcription factor is regulated by Wnt signalling. let-19 and dpy-22, the authors show, encode homologues of MED13 and MED12, components of the Mediator complex, which associates with RNA polymerase to facilitate the activities of various transcription factors. These and other results indicate that, in C. elegans, the Mediator complex regulates not only asymmetric division, as it does in yeast, but also other processes regulated by Wnt signalling.
The stability and intracellular localisation of ß-catenin (also known as Armadillo in flies) is a central control point in Wnt signalling. Now, on p. 1819, Hayward and co-workers report a surprising way in which the activity of Armadillo/ß-catenin might be regulated – by Notch. Their genetic and biochemical experiments in developing Drosophila and in cultured fly and mouse cells indicate that Notch modulates Wnt signalling – an idea that has previously attracted some controversy – by regulating Armadillo/ß-catenin activity. This regulation occurs independently of the interaction of Notch's active intracellular domain with Suppressor of Hairless, an interaction needed for the transcriptional regulation of many of Notch's previously recognised targets. The researchers suggest that the modulation of Armadillo/ß-catenin activity by Notch both establishes a threshold for Wnt signalling and stringently regulates activated Armadillo/ß-catenin, a function that is relevant to Notch's tumour suppressor role in mouse skin.
