Molecules making waves in axon guidance

The exquisite complexity of neural connectivity depends upon the precise navigation of axons to their targets in the developing nervous system. How is this achieved? Signals from the environment are assumed to impinge on growth cone receptors and thus steer axons in the right direction, but the molecular details of this process have been largely unknown. Recently, impressive progress has been made in identifying families of molecules that may underlie this process. Candidate diffusible guidance molecules include the netrins and semaphorins, whereas membrane-associated Eph receptors and their ligands are proposed to influence guidance by cellcell contact. Characterization of receptors for some of these molecules now promises the dissection of the signalling pathways that dictate axonal responses to pathfinding cues

Sonic Hedgehog Is a Chemoattractant for Midbrain Dopaminergic Axons

Midbrain dopaminergic axons project from the substantia nigra (SN) and the ventral tegmental area (VTA) to rostral target tissues, including the striatum, pallidum, and hypothalamus. The axons from the medially located VTA project primarily to more medial target tissues in the forebrain, whereas the more lateral SN axons project to lateral targets including the dorsolateral striatum. This structural diversity underlies the distinct functions of these pathways. Although a number of guidance cues have been implicated in the formation of the distinct axonal projections of the SN and VTA, the molecular basis of their diversity remains unclear. Here we investigate the molecular basis of structural diversity in mDN axonal projections. We find that Sonic Hedgehog (Shh) is expressed at a choice point in the course of the rostral dopaminergic projections. Furthermore, in midbrain explants, dopaminergic projections are attracted to a Shh source. Finally, in mice in which Shh signaling is inactivated during late neuronal development, the most medial dopaminergic projections are deficient

Extra-Renal Elimination of Uric Acid via Intestinal Efflux Transporter BCRP/ABCG2

Urinary excretion accounts for two-thirds of total elimination of uric acid and the remainder is excreted in feces. However, the mechanism of extra-renal elimination is poorly understood. In the present study, we aimed to clarify the mechanism and the extent of elimination of uric acid through liver and intestine using oxonate-treated rats and Caco-2 cells as a model of human intestinal epithelium. In oxonate-treated rats, significant amounts of externally administered and endogenous uric acid were recovered in the intestinal lumen, while biliary excretion was minimal. Accordingly, direct intestinal secretion was thought to be a substantial contributor to extra-renal elimination of uric acid. Since human efflux transporter BCRP/ABCG2 accepts uric acid as a substrate and genetic polymorphism causing a decrease of BCRP activity is known to be associated with hyperuricemia and gout, the contribution of rBcrp to intestinal secretion was examined. rBcrp was confirmed to transport uric acid in a membrane vesicle study, and intestinal regional differences of expression of rBcrp mRNA were well correlated with uric acid secretory activity into the intestinal lumen. Bcrp1 knockout mice exhibited significantly decreased intestinal secretion and an increased plasma concentration of uric acid. Furthermore, a Bcrp inhibitor, elacridar, caused a decrease of intestinal secretion of uric acid. In Caco-2 cells, uric acid showed a polarized flux from the basolateral to apical side, and this flux was almost abolished in the presence of elacridar. These results demonstrate that BCRP contributes at least in part to the intestinal excretion of uric acid as extra-renal elimination pathway in humans and rats

Directed Neural Differentiation of Mouse Embryonic Stem Cells Is a Sensitive System for the Identification of Novel Hox Gene Effectors

The evolutionarily conserved Hox family of homeodomain transcription factors plays fundamental roles in regulating cell specification along the anterior posterior axis during development of all bilaterian animals by controlling cell fate choices in a highly localized, extracellular signal and cell context dependent manner. Some studies have established downstream target genes in specific systems but their identification is insufficient to explain either the ability of Hox genes to direct homeotic transformations or the breadth of their patterning potential. To begin delineating Hox gene function in neural development we used a mouse ES cell based system that combines efficient neural differentiation with inducible Hoxb1 expression. Gene expression profiling suggested that Hoxb1 acted as both activator and repressor in the short term but predominantly as a repressor in the long run. Activated and repressed genes segregated in distinct processes suggesting that, in the context examined, Hoxb1 blocked differentiation while activating genes related to early developmental processes, wnt and cell surface receptor linked signal transduction and cell-to-cell communication. To further elucidate aspects of Hoxb1 function we used loss and gain of function approaches in the mouse and chick embryos. We show that Hoxb1 acts as an activator to establish the full expression domain of CRABPI and II in rhombomere 4 and as a repressor to restrict expression of Lhx5 and Lhx9. Thus the Hoxb1 patterning activity includes the regulation of the cellular response to retinoic acid and the delay of the expression of genes that commit cells to neural differentiation. The results of this study show that ES neural differentiation and inducible Hox gene expression can be used as a sensitive model system to systematically identify Hox novel target genes, delineate their interactions with signaling pathways in dictating cell fate and define the extent of functional overlap among different Hox genes

beta-Carotene is incorporated or mobilized along with triglycerides in bovine adipose tissue in response to insulin or epinephrine

Pasture fed cattle ingest substantial amounts of beta-carotene (beta-C). Not all of the carotenoid compound is transformed into vitamin A, but the surplus is deposited in adipose tissue (AT). The mechanisms of beta-C incorporation and mobilization are unknown. Two experiments were conducted using explants from bovine AT cultured in vitro. First, beta-C incorporation by explants from three animals was examined with different beta-C concentrations (0, 1, 5 and 20 mu m) and different times of incubation (every 5 h up to 25 h). The data showed a significant increase of beta-C concentration in explants only for 20 mu m beta-C. Secondly, effects of insulin and epinephrine on beta-C and triglyceride (TG) contents of explants were studied. Explants from six animals were incubated with either hormone and 0 or 20 mu m beta-C for 20 h. Both TG and beta-C contents were affected positively by insulin and negatively by epinephrine. Interestingly, changes in ratios of beta-C/TG (hormone vs. control) were similar (1.7 x 10(-3) and 1.8 x 10(-3)), respectively, for insulin and epinephrine, indicating that beta-C level is directly related to TG content. We also report the presence of mRNA for beta-C 15, 15' oxygenase in bovine AT. The in vitro culture system using explants from bovine AT is a promising model to investigate factors that might affect the accumulation and metabolism of beta-C