Javier Capdevila

Mechanisms of Left–Right Determination in Vertebrates

We would like to apologize for the impossibility of citing all relevant references due to space constraints. The authors would like to thank M. Brueckner, C. Rodriguez-Esteban, and J. Yost for stimulating discussions that have helped to shape some of the ideas presented in this review. K. J. V. is a Genetics Institute Fellow of the Life Sciences Research Foundation, and research in C. T.s laboratory on this topic is supported by a grant from the NIH. Research on L/R development in J. C. I. B.s laboratory is supported by grants from the NIH and the G. Harold and Leila Y. Mathers Charitable Foundation.

WNT Signals Control FGF-Dependent Limb Initiation and AER Induction in the Chick Embryo

A regulatory loop between the fibroblast growth factors FGF-8 and FGF-10 plays a key role in limb initiation and AER induction in vertebrate embryos. Here, we show that three WNT factors signaling through beta-catenin act as key regulators of the FGF-8/FGF-10 loop. The Wnt-2b gene is expressed in the intermediate mesoderm and the lateral plate mesoderm in the presumptive chick forelimb region. Cells expressing Wnt-2b are able to induce Fgf-10 and generate an extra limb when implanted into the flank. In the presumptive hindlimb region, another Wnt gene, Wnt-8c, controls Fgf-10 expression, and is also capable of inducing ectopic limb formation in the flank. Finally, we also show that the induction of Fgf-8 in the limb ectoderm by FGF-10 is mediated by the induction of Wnt-3a. Thus, three WNT signals mediated by beta-catenin control both limb initiation and AER induction in the vertebrate embryo.

Control of Vertebrate Limb Outgrowth by the Proximal Factor Meis2 and Distal Antagonism of BMPs by Gremlin

The mechanisms controlling growth and patterning along the proximal-distal axis of the vertebrate limb are yet to be understood. We show that restriction of expression of the homeobox gene Meis2 to proximal regions of the limb bud is essential for limb development, since ectopic Meis2 severely disrupts limb outgrowth. We also uncover an antagonistic relationship between the secreted factors Gremlin and BMPs required to maintain the Shh/FGF loop that regulates distal outgrowth. These proximal and distal factors have coordinated activities: Meis2 can repress distal genes, and Bmps and Hoxd genes restrict Meis2 expression to the proximal limb bud. Moreover, combinations of BMPs and AER factors are sufficient to distalize proximal limb cells. Our results unveil a novel set of proximal-distal regulatory interactions that establish and maintain outgrowth of the vertebrate limb.

The novel Cer-like protein Caronte mediates the establishment of embryonic left-right asymmetry.

In the chick embryo, left–right asymmetric patterns of gene expression in the lateral plate mesoderm are initiated by signals located in and around Hensens node. Here we show that Caronte (Car), a secreted protein encoded by a member of the Cerberus/Dan gene family, mediates the Sonic hedgehog (Shh)-dependent induction of left-specific genes in the lateral plate mesoderm. Car is induced by Shh and repressed by fibroblast growth factor-8 (FGF-8). Car activates the expression of Nodal by antagonizing a repressive activity of bone morphogenic proteins (BMPs). Our results define a complex network of antagonistic molecular interactions between Activin, FGF-8, Lefty-1, Nodal, BMPs and Car that cooperate to control left–right asymmetry in the chick embryo.

The function of engrailed and the specification of Drosophila wing pattern.

The adult Drosophila wing (as the other appendages) is subdivided into anterior and posterior compartments that exhibit characteristic patterns. The engrailed (en) gene has been proposed to be paramount in the specification of the posterior compartment identity. Here, we explore the adult en function by targeting its expression in different regions of the wing disc. In the anterior compartment, ectopic en expression gives rise to the substitution of anterior structures by posterior ones, thus demonstrating its role in specification of posterior patterns. The en-expressing cells in the anterior compartment also induce high levels of the hedgehog (hh) and decapentaplegic (dpp) gene products, which results in local duplications of anterior patterns. Besides, hh is able to activate en and the engrailed-related gene invected (inv) in this compartment. In the posterior compartment we find that elevated levels of en product result in partial inactivation of the endogenous en and inv genes, indicating the existence of a negative autoregulatory mechanism. We propose that en has a dual role: a general one for patterning of the appendage, achieved through the activation of secreted proteins like hh and dpp, and a more specific one, determining posterior identity, in which the inv gene may be implicated.

Extracellular modulation of the Hedgehog, Wnt and TGF-β signalling pathways during embryonic development

Localized embryonic expression of members of the Hedgehog, Wnt and TGF-beta families of secreted factors has been shown to organize pattern and provide positional information in many developing systems. Recently, several extracellular factors have been described that act either as facilitators or inhibitors of the activities of those secreted proteins. The variety of molecular strategies involved in the extracellular modulation of signalling activities in the embryo underscores the importance of maintaining a tight spatial and temporal control of the activities of organizing centers during development.

23 – Perspectives on the Evolutionary Origin of Tetrapod Limbs

The study of the origin and evolution of the tetrapod limb has benefited enor- mously from the confluence of molecular and paleontological data. In the last two decades, our knowledge of the basic molecular mechanisms that control limb development has grown exponen- tially, and developmental biologists now have the possibility of combining molecular data with many available descriptions of the fossil record of vertebrate fins and limbs. This synthesis of developmental and evolutionary biology has the potential to unveil the sequence of molecular changes that culminated in the adoption of the basic tetrapod limb plan. J. Exp. Zool. (Mol. Dev. Evol.) 288:287-303, 2000.

Knowing left from right: the molecular basis of laterality defects

The apparent symmetry of the vertebrate body conceals profound asymmetries in the development and placement of internal organs. Asymmetric organ development is controlled in part by genes expressed asymmetrically in the early embryo, and alterations in the activities of these genes can result in severe defects during organogenesis. Recently, data from different vertebrates have allowed researchers to put forward a model of genetic interactions that explains how asymmetric patterns of gene expression in the early embryo are translated into spatial patterns of asymmetric organ development. This model helps us to understand the molecular basis of a number of congenital malformations in humans.

Perspectives on the evolutionary origin of tetrapod limbs.

The study of the origin and evolution of the tetrapod limb has benefited enormously from the confluence of molecular and paleontological data. In the last two decades, our knowledge of the basic molecular mechanisms that control limb development has grown exponentially, and developmental biologists now have the possibility of combining molecular data with many available descriptions of the fossil record of vertebrate fins and limbs. This synthesis of developmental and evolutionary biology has the potential to unveil the sequence of molecular changes that culminated in the adoption of the basic tetrapod limb plan.

CHAPTER 254 – Control of Left-Right (L/R) Determination in Vertebrates by the Hedgehog Signaling Pathway

Although the bodies of most vertebrate animals appear to be almost perfectly symmetrical, profound left-right (L/R) asymmetries exist that are clearly demonstrated by the specific disposition of internal organs. At another level, there is also asymmetry in the activity of the brain with specific functions performed by each individual brain hemisphere, resulting in asymmetries in locomotor functions (among which being right- or left-handed is only one well-known manifestation). The normal disposition of organs is called situs solitus, and situs inversus refers to a complete or near-complete mirror-image reversal of organ disposition. Right or left isomerism usually refers to situations where individual organs are bilaterally symmetric. Although the mechanisms that break the initial symmetry may be very different in chick and mouse embryos, the basic role of HH signaling as a left determinant appears to be conserved during evolution, as is the left-specific expression of the Nodal gene. Without a doubt, the characterization of additional components of HH signaling pathways and of their roles in the early vertebrate embryo will still provide very valuable information about the mechanisms that control L/R determination during early embryogenesis.