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R D Riddle

Publications and source records attributed to R D Riddle.

10 recordsLinked to original sources
PubMed2000

The transcription factor Lmx1b maintains Wnt1 expression within the isthmic organizer.

Cells in the caudal mesencephalon and rostral metencephalon become organized by signals emanating from the isthmus organizer (IsO). The IsO is associated with the isthmus, a morphological constriction of the neural tube which eventually defines the mesencephalic/ metencephalic boundary (MMB). Here we report that the transcription factor Lmx1b is expressed and functions in a distinct region of the IsO. Lmx1b expression is maintained by the glycoprotein Fgf8, a signal capable of mediating IsO signaling. Lmx1b, in turn, maintains the expression of the secreted factor Wnt1. Our conclusions are substantiated by the following: (i) Lmx1b mRNA becomes localized to the isthmus immediately after Fgf8 initiation, (ii) Wnt1 expression is localized to the Lmx1b expression domain, but with slightly later kinetics, (iii) Fgf8-soaked beads generate similar domains of expression for Lmx1b and Wnt1 and (iv) retroviral-mediated expression of Lmx1b (Lmx1b/RCAS) maintains Wnt1 expression in the mesencephalon. Ectopic Lmx1b is insufficient to alter the expression of a number of other genes expressed at the IsO, suggesting that it does not generate a new signaling center. Instead, if we allow Lmx1b/RCAS-infected brains to develop longer, we detect changes in mesencephalic morphology. Since both ectopic and endogenous Lmx1b expression occurs in regions of the isthmus undergoing morphological changes, it could normally play a role in this process. Furthermore, a similar phenotype is not observed in Wnt1/RCAS-infected brains, demonstrating that ectopic Wnt1 is insufficient to mediate the effect of ectopic Lmx1b in our assay. Since Wnt1 function has been linked to the proper segregation of mesencephalic and metencephalic cells, we suggest that Lmx1b and Wnt1 normally function in concert to affect IsO morphogenesis.

Animals↗
PubMed1996

Sonic hedgehog regulates patterning in early embryos.

Recently, a new family of genes, homologues of the Drosophila segment polarity gene hedgehog, has been cloned in vertebrates. One of them, Sonic hedgehog, is expressed in tissues that are known to be inductive centres for patterning early embryos, implicating Sonic hedgehog as an important signal in development. Functional experiments have demonstrated that Sonic hedgehog acts as a signalling molecule in patterning the anterior-posterior axis of the limb. By misexpression of Sonic hedgehog we show that Sonic hedgehog induces expression of Hoxd genes, known to be involved in patterning of the anterior-posterior axis of the limb, and Bmp-2, which might act as a secondary signal. We also demonstrate that Sonic hedgehog is not sufficient for these inductions. In fact, a signal from the mesoderm, Sonic hedgehog, and a signal from the ectoderm, Fgf-4, are required for the induction of Hoxd genes and Bmp-2.

Animals↗
PubMed1995

Induction of the LIM homeobox gene Lmx1 by WNT7a establishes dorsoventral pattern in the vertebrate limb.

During vertebrate limb development, the ectoderm directs the dorsoventral patterning of the underlying mesoderm. To define the molecular events involved in this process, we have analyzed the function of WNT7a, a secreted factor expressed in the dorsal ectoderm, and LMX1, a LIM homeodomain transcription factor expressed in the dorsal mesenchyme. Ectopic expression of Wnt7a is sufficient to induce and maintain Lmx1 expression in limb mesenchyme, both in vivo and in vitro. Ectopic expression of Lmx1 in the ventral mesenchyme is sufficient to generate double-dorsal limbs. Thus, the dorsalization of limb mesoderm appears to involve the WNT7a-mediated induction of Lmx1 in limb mesenchymal cells.

Animals↗
PubMed1994

Ectopic expression of Sonic hedgehog alters dorsal-ventral patterning of somites.

Differentiation of somites into sclerotome, dermatome, and myotome is controlled by a complex set of inductive interactions. The ability of axial midline tissues, the notochord and floor plate, to induce sclerotome has been well documented and has led to models in which ventral somite identity is specified by signals derived from the notochord and floor plate. Herein, we provide evidence that Sonic hedgehog, a vertebrate homolog of the Drosophila segment polarity gene hedgehog, is a signal produced by the notochord and floor plate that directs ventral somite differentiation. Sonic hedgehog is expressed in ventral midline tissues at critical times during somite specification and has the ability, when ectopically expressed, to enhance the formation of sclerotome and antagonize the development of dermatome.

Amino Acid Sequence↗
PubMed1994

Structure and regulation of the chicken erythroid delta-aminolevulinate synthase gene.

Erythroid cells regulate heme biosynthesis in a manner that is distinct from all other cell types. While heme negatively regulates the synthesis of the housekeeping delta-aminolevulinate synthase (ALAS-N) in all non-erythroid cells, the expression of an erythroid-specific isozyme (ALAS-E) is developmentally regulated in red blood cells. As a first step towards understanding the molecular basis for the transcriptional regulation of ALAS-E during erythropoiesis, we cloned and characterized the chicken ALAS-E locus. This gene spans 18 kbp and is composed of eleven exons. The intron/exon structure of erythroid ALAS was found to be conserved among several vertebrate species. Direct RNA sequencing identified a 5' untranslated region that is derived from two continuous exons and is predicted to form a very stable stem-loop structure that bears resemblance to the ferritin iron-responsive element. Tissue-specific expression of the ALAS-E gene was analyzed by transient transfection assays in hematopoietic cells of both erythroid and non-erythroid origins. These experiments identified distal (-784 to -505 bp) and proximal (-155 to +21 bp) promoter elements which are required for high level, erythroid-specific transcription.

5-Aminolevulinate Synthetase↗
PubMed1994

Mechanisms of limb patterning.

The development of the vertebrate limb requires the coordinated action of multiple signals to achieve the proper arrangement of adult tissues. Recently, several molecules have been identified which play central roles in patterning of the limb bud. Sonic hedgehog, a homolog of the Drosophila segment polarity gene hedgehog, is likely to regulate anterior/posterior pattern formation. FGF-2 and FGF-4, members of the fibroblast growth factor family, have been shown to provide important signals for limb bud outgrowth and to indirectly regulate proximal/distal patterning. Some candidate effectors of the activity of Sonic hedgehog and of FGFs are known, including members of the clustered Hox genes.

Animals↗
PubMed1994

Sonic hedgehog: a key mediator of anterior-posterior patterning of the limb and dorso-ventral patterning of axial embryonic structures.

Sonic hedgehog is expressed in several sites during embryogenesis which are known to be important in directing the development of neighbouring tissues, including Hensen's node, the notochord, the floor plate of the neural tube, and the posterior of the limb bud. A unity in signalling mechanisms utilized by these inducers was first indicated because they all can provide a source of limb polarizing activity, assayed by grafting into the anterior of a limb bud. The hypothesis that they share a common signal is substantiated by the fact that they all express Sonic. Moreover, ectopic expression of Sonic in vivo suggests that it is responsible for the polarizing activity of the ZPA and plays an important role in dorso-ventral patterning of the spinal cord. The isolation of Sonic heralds a new era in the investigation of the molecular mechanisms of these key inductive interactions in vertebrate development.

Amino Acid Sequence↗
PubMed1993

Sonic hedgehog mediates the polarizing activity of the ZPA.

The zone of polarizing activity (ZPA) is a region at the posterior margin of the limb bud that induces mirror-image duplications when grafted to the anterior of a second limb. We have isolated a vertebrate gene, Sonic hedgehog, related to the Drosophila segment polarity gene hedgehog, which is expressed specifically in the ZPA and in other regions of the embryo, that is capable of polarizing limbs in grafting experiments. Retinoic acid, which can convert anterior limb bud tissue into tissue with polarizing activity, concomitantly induces Sonic hedgehog expression in the anterior limb bud. Implanting cells that express Sonic hedgehog into anterior limb buds is sufficient to cause ZPA-like limb duplications. Like the ZPA, Sonic hedgehog expression leads to the activation of Hox genes. Sonic hedgehog thus appears to function as the signal for antero-posterior patterning in the limb.

Amino Acid Sequence↗
PubMed1989

Expression of delta-aminolevulinate synthase in avian cells: separate genes encode erythroid-specific and nonspecific isozymes.

A controversy has existed in the literature for the past several years regarding the number of vertebrate genes encoding the mitochondrial protein that initiates the first step in heme biosynthesis, delta-aminolevulinate synthase [ALAS; succinyl-CoA: glycine C-succinyltransferase (decarboxylating), EC 2.3.1.37]. By analysis of chicken ALAS cDNA clones isolated from both liver and erythroid cells, we show that at least two separate genes encode ALAS mRNAs. These experiments show that (i) two different genes encode the ALAS isozymes found in erythroid and in liver tissues, and (ii) while the product of the erythroid gene (ALASE) is expressed exclusively in erythroid cells, the hepatic form of the enzyme is expressed ubiquitously, suggesting that this is the nonspecific form (ALASN) found in all chicken tissues.

5-Aminolevulinate Synthetase↗