Showing papers by "Douglas A. Melton published in 1992"

A truncated activin receptor inhibits mesoderm induction and formation of axial structures in Xenopus embryos

Abstract: Activins can induce mesoderm in embryonic explants and have been proposed as the natural inducer in Xenopus. A mutant activin receptor that inhibits activin signalling is used to show that activin is required for the induction of mesoderm in vivo and the patterning of the embryonic body plan. Blocking the activin signal transduction pathway also reveals autonomous induction of a neural marker and unmasks a relationship between activin and fibroblast growth factor.

Specificity of antisense oligonucleotides in vivo.

Abstract: Antisense oligonucleotides are widely used as inhibitors of gene expression in cultured cells and have been proposed as potential therapeutic agents, but it is not known to what extent they are specific for their intended target RNAs. Statistical considerations indicate that if oligonucleotides can form hybrids with mRNA molecules in vivo by means of short or imperfect regions of complementarity, then the specificity of oligonucleotides as antisense reagents will be greatly compromised. We have used Xenopus oocytes as a model system in which to investigate the potential specificity of antisense oligonucleotides in vivo. We injected perfect and partially matched antisense oligonucleotides into oocytes and measured the resulting degradation of the target RNA in each case. On the basis of the extent to which antisense oligonucleotides can cause cleavage of RNAs at imperfectly matched target sites, we conclude that in this system it is probably not possible to obtain specific cleavage of an intended target RNA without also causing at least the partial destruction of many nontargeted RNAs.

Vegetal messenger RNA localization directed by a 340-nt RNA sequence element in Xenopus oocytes

Abstract: Contained within a single cell, the fertilized egg, is information that will ultimately specify the entire organism. During early embryonic cleavages, cells acquire distinct fates and their differences in developmental potential might be explained by localization of informational molecules in the egg. The mechanisms by which Vg1 RNA, a maternal mRNA, is translocated to the vegetal pole of Xenopus oocytes may indicate how developmental signals are localized. Data presented here show that a 340-nucleotide localization signal present in the 39 untranslated region of Vg1 RNA is sufficient to direct RNA localization to the vegetal pole.

Involvement of p21ras in Xenopus mesoderm induction.

Abstract: During early vertebrate embryogenesis, mesoderm is specified by a signal emanating from prospective endoderm. This signal can respecify Xenopus prospective ectoderm as mesoderm, and can be mimicked by members of the fibroblast growth factor and transforming growth factor-beta families. In other systems, the p21c-ras proto-oncogene product has been implicated in signal transduction for various polypeptide growth factors. We report here that a dominant inhibitory ras mutant blocks the mesoderm-inducing activity of fibroblast growth factor and activin, as well as the endogenous inducing activity of prospective endoderm. A constitutively active ras mutant partially mimics these activities. These results indicate that p21ras may have a central role in the transduction of the mesoderm inductive signal. Basic fibroblast growth factor and activin have emerged as candidates for endogenous mesoderm-inducing molecules. The character of the mesoderm induced by these two factors is overlapping but distinct when assessed both by histological and molecular criteria. The signal transduction pathways used during induction by these factors are unknown. We used messenger RNA microinjection of Xenopus eggs to express a dominant inhibitory mutant ras, p21(Asn 17)Ha-ras, in cells competent to respond to inducing factors to examine the role of p21ras in this response. This mutant, which has a reduced affinity for GTP relative to GDP, blocks a variety of mitogenic signals in 3T3 fibroblasts as well as the differentiation of pheochromocytoma cells in response to nerve growth factor.

Embryonic expression and functional analysis of a Xenopus activin receptor.

Abstract: We report the isolation and characterization of a Xenopus activin receptor (XAR1). The amino acid sequence of this protein shows extensive homology with a murine activin receptor. The mRNA is expressed maternally and is ubiquitously distributed during the early stages of embryogenesis. Consistent with a possible role in mesoderm induction and patterning, interference with the normal expression of the receptor by overexpression in the early embryo results in the formation of ectopic dorsal axial structures. During neurulation the XAR1 mRNA is expressed predominantly in the presumptive brain and spinal cord, suggesting an additional function for XAR1 in neurogenesis. © 1992 Wiley-Liss, Inc.