Search PubMed⌕ Search

Biomedical subjects

J L Christian

Publications and source records attributed to J L Christian.

35 records · Page 2Linked to original sources
PubMed1995

Xwnt-8b: a maternally expressed Xenopus Wnt gene with a potential role in establishing the dorsoventral axis.

In amphibian embryos, establishment of dorsal-ventral asymmetry is believed to involve dorsal-ventral differences in vegetally derived mesoderm-inducing signals and/or differences in the competence of animal hemisphere (ectodermal) cells to respond to these signals. Previous studies have shown that certain Wnt proteins can generate an ectopic dorsal axis when misexpressed, and that they do so by modifying the response of ectodermal cells to inducers. None of these Wnt proteins are expressed at an appropriate time to do so in vivo. In this study, we describe the isolation and characterization of a full length cDNA for the Xenopus Wnt gene, Xwnt-8b, whose biological activity and expression pattern suggest that it may be involved in establishment of the dorsoventral axis. Both maternal and zygotic Xwnt-8b transcripts undergo alternative splicing to generate mRNAs which encode two different forms of Xwnt-8b protein. During early cleavage stages Xwnt-8b transcripts are confined primarily to animal hemisphere blastomeres, while zygotically derived Xwnt-8b transcripts are restricted almost exclusively to a band of cells in the prospective forebrain of neurula and tailbud stage embryos. Ectopically expressed Xwnt-8b can completely rescue dorsal development of embryos ventralized by exposure to ultraviolet light, and can induce a complete secondary axis in wild-type embryos. Axis induction is observed only if Xwnt-8b is supplied prior to the onset of zygotic gene transcription. This biological activity, together with the presence of maternal Xwnt-8b transcripts in cells that will be induced to form the dorsal mesoderm, is consistent with the possibility that Xwnt-8b may be the endogenous agent that establishes asymmetry in the response of ectodermal cells to mesoderm-inducing signals, thereby initiating dorsal development.

Alternative Splicing↗
PubMed1993

Interactions between Xwnt-8 and Spemann organizer signaling pathways generate dorsoventral pattern in the embryonic mesoderm of Xenopus.

This study analyzes the hierarchy of signals that spatially restrict expression of Xenopus Xwnt-8 to mesodermal cells outside of the Spemann organizer field and examines the potential role that endogenous Xwnt-8 may play in dorsoventral patterning of the embryonic mesoderm. The effects of ectopic introduction of a Nieuwkoop center-like activity or of ectopic expression of goosecoid, on the distribution of endogenous Xwnt-8 transcripts were analyzed. The results of these studies are consistent with the hypothesis that maternally derived signals from the Nieuwkoop center function to positively regulate expression of the homeo box gene goosecoid in Spemann organizer cells, leading to a subsequent repression of Xwnt-8 expression in these cells. This exclusion of Xwnt-8 from cells of the organizer field may be important for normal dorsal development, in that ectopic expression of Xwnt-8 in organizer cells after the midblastula stage, by injection of plasmid DNA, ventralizes the fate of these cells. This is distinct from the previously observed dorsalizing effect of Xwnt-8 when expressed prior to the midblastula stage by injection of RNA. The effects of plasmid-derived Xwnt-8 on isolated blastula animal cap ectoderm were also analyzed. Expression of Xwnt-8 in animal pole ectoderm after the midblastula stage ventralizes the response of dorsal animal pole cells to activin and allows naive ectodermal cells to differentiate as ventral mesoderm in the absence of added growth factors. Collectively, these data are consistent with the hypothesis that Xwnt-8 plays a role in the mesodermal differentiation of ventral marginal zone cells during normal development. Furthermore, endogenous Xwnt-8 may ventralize the response of lateral mesodermal cells to dorsalizing signals from the organizer, thus contributing to the graded nature of the final body pattern.

Activins↗
PubMed1993

Xwnt-5A: a maternal Wnt that affects morphogenetic movements after overexpression in embryos of Xenopus laevis.

To contribute to an understanding of the roles and mechanisms of action of Wnts in early vertebrate development, we have characterized the normal expression of Xenopus laevis Wnt-5A, and investigated the consequences of misexpression of this putative signalling factor. Xwnt-5A transcripts are expressed throughout development, and are enriched in both the anterior and posterior regions of embryos at late stages of development, where they are found primarily in ectoderm, with lower levels of expression in mesoderm. Overexpression of Xwnt-5A in Xenopus embryos leads to complex malformations distinct from those achieved by ectopic expression of Xwnts -1, -3A, or -8. This phenotype is unlikely to result from Xwnt-5A acting as an inducing agent, as overexpression of Xwnt-5A does not rescue dorsal structures in UV-irradiated embryos, does not induce mesoderm in blastula caps, and Xwnt-5A does not alter the endogenous patterns of expression of goosecoid, Xbra, or Xwnt-8. To pursue whether Xwnt-5A has the capacity to affect morphogenetic movements, we investigated whether overexpression of Xwnt-5A alters the normal elongation of blastula cap explants induced by activin. Intriguingly, Xwnt-5A blocks the elongation of blastula caps in response to activin, without blocking the differentiation of either dorsal or ventral mesoderm within these explants. The data are consistent with Xwnt-5A having the potential activity of modifying the morphogenetic movements of tissues.

Amino Acid Sequence↗
PubMed1993

Dissecting Wnt signalling pathways and Wnt-sensitive developmental processes through transient misexpression analyses in embryos of Xenopus laevis.

We review evidence that Xenopus Wnts (Xwnts) have activities consistent with their hypothesized roles as secreted signalling factors involved in multiple developmental processes. Transient misexpression of different Xwnts has distinct effects upon early development, and upon the formation of tissues in UV-irradiated embryos. Misexpression of Xwnts also has distinct effects on the in vitro differentiation of blastula cap explants. Cellular responses to Xwnt signals include changes in gap junctional permeability, altered responsiveness to growth factors, and possibly changes in cell adhesion. Current data suggest that a maternal Xwnt- or noggin-like activity is involved in the Nieuwkoop center activity during mesoderm induction, that Xwnt-8 participates in a pathway of differentiation as ventral mesoderm, and that Xwnt-5A is a potential modulator of morphogenetic movements.

Animals↗
PubMed1992

Xwnt-8 modifies the character of mesoderm induced by bFGF in isolated Xenopus ectoderm.

In Xenopus, growth factors of the TGF-beta, FGF and Wnt oncogene families have been proposed to play a role in generating embryonic pattern. In this paper we examine potential interactions between the bFGF and Xwnt-8 signaling pathways in the induction and dorsal-ventral patterning of mesoderm. Injection of Xwnt-8 mRNA into 2-cell Xenopus embryos does not induce mesoderm formation in animal cap ectoderm isolated from these embryos at the blastula stage, but alters the response of this tissue to mesoderm induction by bFGF. While animal cap explants isolated from non-injected embryos differentiate to form ventral types of mesoderm and muscle in response to bFGF, explants from Xwnt-8 injected embryos form dorsal mesodermal and neural tissues in response to the same concentration of bFGF, even if the ectoderm is isolated from the prospective ventral sides of embryos or from UV-ventralized animals. Our results support a model whereby dorso-ventral mesodermal patterning can be attained by a single mesoderm inducing agent, possibly bFGF, which is uniformly distributed across the prospective dorsal-ventral axis, and which acts in concert with a dorsally localized signal, possibly a Wnt protein, which either alters the response of ectoderm to induction or modifies the character of mesoderm after its induction.

Animals↗
PubMed1992

Synergistic principles of development: overlapping patterning systems in Xenopus mesoderm induction.

The first inductive event in Xenopus development establishes the mesoderm at the equator of the developing embryo. As part of this process, the dorsal-ventral and anterior-posterior axes of the embryo are initially established. A number of signalling molecules which may play a role in mesodermal induction and patterning have been identified in the last several years, including members of the FGF, TGF-beta and Wnt gene families. A variety of experiments, using either purified factors or injection of RNA encoding these factors, have added to the wealth of classical embryological experimental data collected over the last century. We have synthesized some recent results with the classical data to provide a framework for examining the process of mesoderm induction, and to formulate putative roles for some of the different factors. We incorporate these ideas into a working model of mesoderm induction that provides a basis for future experimental directions. Finally, we suggest that mesoderm induction may not be a discrete set of well separated events, but instead may be a process involving partially overlapping signals that produce the same pattern.

Animals↗
PubMed1991

Injected Wnt RNA induces a complete body axis in Xenopus embryos.

Studies in Xenopus have shown that growth factors of the TGF beta and Wnt oncogene families can mimic aspects of dorsal axis formation. Here we directly compare the inductive properties of two Wnt proteins by injecting synthetic mRNA into developing embryos. The results show that Wnt-1 and Xwnt-8 can induce a new and complete dorsal axis and can rescue the development of axis-deficient, UV-irradiated embryos. In contrast, activin mRNA injection induces only a partial dorsal axis that lacks anterior structures. These studies demonstrate that the mechanism of Wnt-induced axis duplication results from the creation of an independent Spemann organizer. The relationship between the properties of the endogenous dorsal inducer and the effects of Wnts and activins is discussed.

Activins↗
PubMed1991

Effect of wnt-1 and related proteins on gap junctional communication in Xenopus embryos.

The proto-oncogene wnt-1 (previously referred to as int-1) is thought to be important in embryonic pattern formation although its mechanisms of action are unknown. Premature and increased expression of the Wnt-1 protein, achieved by injection of synthetic wnt-1 RNA into fertilized Xenopus eggs, enhanced gap junctional communication between ventral cells of the developing embryo. This result is consistent with the hypothesis that Wnt proteins activate a receptor-mediated signal transduction pathway and that gap junctional communication can be a target of this pathway. The effects of two Wnt-1-related proteins on gap junctional communication were also investigated: overexpression of Xwnt-8 increased gap junctional coupling in a manner similar to Wnt-1, whereas Xwnt-5A did not. These findings are consistent with the existence of multiple receptors for Wnt proteins.

Animals↗
PubMed1991

Isolation of cDNAs partially encoding four Xenopus Wnt-1/int-1-related proteins and characterization of their transient expression during embryonic development.

To begin to study the functions of the Wnt-1/int-1 gene family during vertebrate development, we have isolated four Xenopus laevis cDNAs encoding the partial sequence of proteins homologous to Wnt-1/int-1. Xwnt-3, Xwnt-4, Xwnt-5A, and Xwnt-8 demonstrate between 35 and 50% amino acid identity with X. laevis Wnt-1/int-1 and most cysteine residues are conserved. Xwnt-4 and Xwnt-3 transcripts are detected only during the neurula through tadpole stages of development. Expression of Xwnt-8 is observable during gastrulation, declines during neurulation, and is undetectable by the tadpole stage of development. Xwnt-5A transcripts are most prevalent in RNA from oocytes and tadpoles, although low level expression is detected at all stages examined. The temporal changes in expression of these transcripts imply a unique role for each Xwnt during embryogenesis.

Amino Acid Sequence↗
PubMed1991

Xwnt-8, a Xenopus Wnt-1/int-1-related gene responsive to mesoderm-inducing growth factors, may play a role in ventral mesodermal patterning during embryogenesis.

In amphibian embryos, formation of the basic body plan depends on positional differences in the mesoderm. Although peptide growth factors involved in mesoderm induction have tentatively been identified, additional signals are required to generate pattern in this tissue. We have isolated a Xenopus cDNA for a Wnt-1 related gene, designated Xwnt-8, which is activated in response to mesoderm-inducing growth factors. Xwnt-8 transcripts are transiently expressed, being most abundant during gastrulation at which time expression is confined primarily to ventral mesodermal cells. Embryos dorsoanteriorized by exposure to lithium exhibit greatly reduced levels of Xwnt-8 mRNA, supporting a correlation between Xwnt-8 expression and a ventral mesodermal cell fate. Surprisingly, ectopic expression of Xwnt-8 in embryos causes a dorsoanterior-enhanced phenotype. These findings suggest that Xwnt-8 may be a secondary signalling agent which is produced in response to mesoderm-inducing factors and is involved in the early steps of mesodermal patterning.

Animals↗
PubMed1990

Overexpression of wild-type and dominant negative mutant vimentin subunits in developing Xenopus embryos.

Vimentin belongs to the diverse multigene family of intermediate filament proteins, each member of which is expressed in a tissue-specific and developmentally regulated pattern. The existence of vimentin filaments has been documented in oocytes, eggs, and early embryos of Xenopus laevis, but the role of these cytoskeletal components remains unknown. To investigate the functions of vimentin during early development in Xenopus, we induced the overexpression of wild-type and deletion mutant subunits in most of the cells of embryos by injecting synthetic RNA into fertilized eggs. Wild-type vimentin subunits, as well as subunits lacking most of the amino-terminal head piece, assembled into normal appearing filaments in vivo. Deletion mutants of the fourth alpha-helical rod domain were assembly incompetent and dominantly inhibited the polymerization of wild-type subunits when both types of subunit were co-expressed in cells. Expression of at least a tenfold excess of wild-type or mutant subunits within cells of embryos did not lead to any detectable morphological or developmental abnormalities, suggesting that the presence and proper regulation of vimentin expression is not essential during the initial stages of embryogenesis in Xenopus.

Amino Acid Sequence↗
PubMed1988

Differential tissue expression and developmental regulation of guanine nucleotide binding regulatory proteins and their messenger RNAs in rat heart.

The expression and developmental regulation of the alpha and beta subunits of the guanine nucleotide binding regulatory proteins, Gi and Go, were examined in rat atria and ventricles. Protein levels were determined by quantitative immunoblot analysis using affinity purified monospecific antibodies. Northern blot and dot blot analyses were used to characterize and quantitate relative amounts of mRNA encoding these G protein subunits. The concentrations of Go alpha, Gi alpha, and beta subunit protein were found to be greater in adult atrial than in adult ventricular membranes (5.2-, 1.5-, and 2.8-fold, respectively). A corresponding 3.4-fold difference in Go alpha mRNA level was also observed, as well as a 1.3-fold difference in Gi alpha-3 mRNA level. No difference was seen between the amount of beta, Gi alpha-1, Gi alpha-2 mRNA in adult atria and adult ventricles. Comparison of neonatal and adult tissues revealed a developmental decrease in ventricular Gi alpha protein and Gi alpha-2 mRNA levels (70 and 47%, respectively). Developmental decreases were also observed in the amount of mRNA encoding beta and Go alpha in ventricles (47 and 61%, respectively), and beta and Gi alpha-2 in atria (40 and 36%, respectively), while a developmental increase in atrial Gi alpha-3 mRNA levels was observed (57%). These results demonstrate differences in the expression of G protein subunits in rat atria and ventricles, as well as regulation of the levels of these subunits during cardiac development.

Adenosine Diphosphate Ribose↗
PubMed1981

Transcapillary Starling pressures in the fetus, newborn, adult, and pregnant adult.

We used the method of membrane osmometry to determine capillary pressure (Pc), plasma protein osmotic pressure (IIp), interstitial protein osmotic pressure (IIif), and interstitial free fluid pressure (Pif) in subcutaneous tissue. These pressures were measured in the sheep and guinea pig fetus, newborn, adult, and pregnant adult. Although there were some similarities, we found major species differences. 1) IIp was a minimum in the fetus and gradually increased to a maximum with adulthood in the guinea pig, whereas IIp attained adult values in the newborn lamb. 2) IIp and Pc did not change with pregnancy in sheep, but, in the guinea pig, IIp and Pc decreased by approximately 30% during pregnancy. 3) The maximum and minimum IIif occurred in the fetal and adult guinea pig, respectively, whereas this was reversed in the sheep. 4) IIif decreased with pregnancy in the guinea pig but not in the sheep. 5) The minimum Pif was found in the newborn of both species. These data suggest that the forces that determine the intravascular-interstitial fluid distribution are different in the fetus, newborn, and adult and also vary among species.

Aging↗
PubMed1980

Transcapillary Starling forces using membrane osmometry.

Membrane osmometry was used to estimate the four transcapillary Starling pressures in subcutaneous tissue of rats, guinea pigs, and dogs. Isolated subcutaneous tissue samples were either placed on a large-pore or small-pore osmometer that measured the interstitial fluid pressure (Pif) and the difference between the interstitial fluid pressure and the interstitial protein osmotic pressure (Pif-pi if), respectively. The colloid osmotic pressure of the interstitial fluid (pi if) was obtained from the difference in these two pressures. A plasma sample placed on the small-pore osmometer yielded the colloid osmotic pressure of the plasma proteins (pi c). Finally the capillary pressure (Pc) was calculated from the three other Starling forces. In the rat, guinea pig, and dog, respectively, the estimated Starling forces were as follows: Pif -2.2, -2.1, and -4.8 mmHg; pi if, 7.3, 4.8, and 4.4 mmHg; pi c, 21.3, 19.5, and 19.2 mmHg; and Pc, 11.8, 12.6, and 10.0 mmHg. A comparison with data obtained in other studies using different methods shows good agreement and strongly supports membrane osmometry as a method for measuring the Starling pressures in subcutaneous tissue.

Animals↗