Intracellular BMP signaling regulation in vertebrates: pathway or network?

Bone morphogenetic proteins (BMPs), members of the TGF-beta superfamily of secreted signaling molecules, have important functions in many biological contexts. They bind to specific serine/threonine kinase receptors, which transduce the signal to the nucleus through Smad proteins. The question of how BMPs can have such diverse effects while using the same canonical Smad pathway has recently come closer to an answer at the molecular level. Nuclear cofactors have been identified that cooperate with the Smads in regulating specific target genes depending on the cellular context. In addition, the pivotal role BMP signaling plays is underscored by the identification of factors that regulate members of this pathway at the cell surface, in the cytoplasm, and in the nucleus. Many of these factors are BMP-inducible and inhibit the BMP pathway, thus establishing negative feedback loops. Members of the BMP-Smad pathway can also physically interact with components of other signaling pathways to establish crosstalk. Finally, there is accumulating evidence that an alternative pathway involving MAP kinases can transduce BMP signals. The evidence and implications of these findings are discussed with an emphasis on early embryonic development of Xenopus and vertebrates.

The Xenopus laevis homeobox gene Xgbx-2 is an early marker of anteroposterior patterning in the ectoderm

In a search for homeobox genes expressed during early Xenopus development, we have isolated a gene which appears to be the Xenopus cognate of the mouse Gbx-2 gene. Expression of Xgbx-2 is first detectable by in situ hybridization at the midgastrula stage when it is predominantly expressed in the dorsolateral ectoderm, with a gap in expression at the dorsal midline. By the end of gastrulation and during neurulation, Xgbx-2 is expressed dorsolaterally in the neural ectoderm and laterally and ventrally in the epidermis with sharp anterior expression borders in both tissues. The anteriormost expression in the neural ectoderm persists throughout the early stages of development, and was mapped to the region of rhombomere 1, with an anterior expression border in the region of the midbrain-hindbrain boundary. Thus Xgbx-2 is expressed anterior to the Hox genes. Xgbx-2 expression is induced by retinoic acid (RA) in animal caps, and RA treatment of whole embryos expands and enhances Xgbx-2 expression in the ectoderm. We suggest a role for Xgbx-2 in establishing the midbrain-hindbrain boundary, which appears to separate early neurectodermal regions expressing genes that are positively and negatively regulated by RA.

Biotransformation and uric acid lowering effect of benzbromarone in patients with liver cirrhosis - evidence for active benzbromarone metabolites?

The disposition of benzbromarone and its uric acid lowering effect were investigated in 8 patients with compensated liver cirrhosis in order to obtain evidence whether dose requirements differ from subjects with normal liver function. Following a single oral dose of 100 mg benzbromarone, the plasma concentrations of the parent drug and the two hydroxylated main metabolites M1 and M2 as well as uric acid were determined up to at least 72 h. All patients were found to be rapid benzbromarone eliminators. In patients 2-8 the extent of systemic availability of benzbromarone, as estimated by the average AUC(0-infinite), was similar to previous observations in healthy individuals, whereas the values of both metabolites M1 and M2 tended to be lower in patients with liver cirrhosis. Cmax of benzbromarone and M1 also were lower in patients, M2 was equivalent to the data in subjects with normal liver function. tmax and the plasma elimination half-life t(1/2) varied within the same range as previously observed in healthy individuals. One patient exhibited much higher values in AUC(0-infinite); and Cmax of benzbromarone and both metabolites, and in addition of the elimination half-life of M1 and M2, whereas the plasma elimination of benzbromarone itself was not delayed. An effect of altered liver function cannot be excluded in this patient. Ten hours after benzbromarone administration the mean plasma uric acid in patients 2-8 was reduced by 31.5% and in patient 1 by 44.2% as compared to pretreatment values. Baseline levels were not regained until 72 h. These data are compatible with a prolonged uric acid lowering effect of an active benzbromarone metabolite. Altogether, the present observations do not suggest dose adjustment to be necessary in patients with compensated liver cirrhosis Child A and B.