Attenuation of bone morphogenetic protein signaling during amphibian limb development results in the generation of stage-specific defects

Loss of noggin1, a classic embryonic inducer gene, in elasmobranchs

Secreted proteins of the Noggin family serve as pivotal regulators of early development and cell differentiation in all multicellular animals, including vertebrates. Noggin1 was identified first among all Noggins. Moreover, it was described as the first known embryonic inducer specifically secreted by the Spemann organizer and capable of inducing a secondary body axis when expressed ectopically. In the classical default model of neural induction, Noggin1 is presented as an antagonist of BMP signalling, playing a role as a neural inducer. Additionally, Noggin1 is involved in the dorsalization of embryonic mesoderm and later controls the differentiation of various tissues, including muscles, bones, and neural crest derivatives. Hitherto, noggin1 was found in all studied vertebrates. Here, we report the loss of noggin1 in elasmobranchs (sharks, rays and skates), which is a unique case among vertebrates. noggin2 and noggin4 retained in this group and studied in the embryos of the grey bamboo shark Chiloscyllium griseum revealed similarities in expression patterns and functional properties with their orthologues described in other vertebrates. The loss of noggin1 in elasmobranchs may be associated with histological features of the formation of their unique internal cartilaginous skeleton, although additional research is required to establish functional connections between these events.

Gene expression analysis of the Xenopus laevis early limb bud proximodistal axis

BACKGROUND

Limb buds develop as bilateral outgrowths of the lateral plate mesoderm and are patterned along three axes. Current models of proximal to distal patterning of early amniote limb buds suggest that two signals, a distal organizing signal from the apical epithelial ridge (AER, Fgfs) and an opposing proximal (retinoic acid [RA]) act early on pattern this axis.

RESULTS

Transcriptional analysis of stage 51 Xenopus laevis hindlimb buds sectioned along the proximal-distal axis showed that the distal region is distinct from the rest of the limb. Expression of capn8.3, a novel calpain, was located in cells immediately flanking the AER. The Wnt antagonist Dkk1 was AER-specific in Xenopus limbs. Two transcription factors, sall1 and zic5, were expressed in distal mesenchyme. Zic5 has no described association with limb development. We also describe expression of two proximal genes, gata5 and tnn, not previously associated with limb development. Differentially expressed genes were associated with Fgf, Wnt, and RA signaling as well as differential cell adhesion and proliferation.

CONCLUSIONS

We identify new candidate genes for early proximodistal limb patterning. Our analysis of RA-regulated genes supports a role for transient RA gradients in early limb bud in proximal-to-distal patterning in this anamniote model organism.

Downregulation of Grem1 expression in the distal limb mesoderm is a necessary precondition for phalanx development

BACKGROUND

The phalanges are the final skeletal elements to form in the vertebrate limb and their identity is regulated by signaling at the phalanx forming region (PFR) located at the tip of the developing digit ray. Here, we seek to explore the relationship between PFR activity and phalanx morphogenesis, which define the most distal limb skeletal elements, and signals associated with termination of limb outgrowth.

RESULTS

As Grem1 is extinguished in the distal chick limb mesoderm, the chondrogenesis marker Aggrecan is up-regulated in the metatarsals and phalanges. Fate mapping confirms that subridge mesoderm cells contribute to the metatarsal and phalanges when subridge Grem1 is down-regulated. Grem1 overexpression specifically blocks chick phalanx development by inhibiting PFR activity. PFR activity and digit development are also disrupted following overexpression of a Gli3 repressor, which results in Grem1 expression in the distal limb and downregulation of Bmpr1b.

CONCLUSIONS

Based on expression and fate mapping studies, we propose that downregulation of Grem1 in the distal limb marks the transition from metatarsal to phalanx development. This suggests that downregulation of Grem1 in the distal limb mesoderm is necessary for phalanx development. Grem1 downregulation allows for full PFR activity and phalanx progenitor cell commitment to digit fate.

THE FORELIMBS OF ALVAREZSAUROIDEA (DINOSAURIA: THEROPODA): INSIGHT FROM EVOLUTIONARY TERATOLOGY

Alvarezsauroidea (Tetanurae) are non-avian theropod dinosaurs whose forelimb evolution is characterised by overdevelopment of digit I, at the expense of the other two digits, complemented by a drastic forelimb shortening in derived species (Parvicursorinae). These variations are recognised as evolutionary developmental anomalies. Evolutionary teratology hence leads to a double diagnosis with 1) macrodactyly of digit I and microdactyly of digits II and III, plus 2) anterior micromelia. The teratological macrodactyly/microdactyly coupling evolved first. Developmental mechanisms disturbing limb proportion are thought to be convergent with those of other Tetanurae (Tyrannosauridae, Carcharodontosauridae). As for the manual anomalies, both are specific to Alvarezsauroidea (macrodactyly/microdactyly) and inherited (digit loss/reduction). While considering the frame-shift theory, posterior digits develop before the most anterior one. There would therefore be a decrease in the area devoted to digits II (condensation 3) and III (condensation 4), in connection with the Shh signalling pathway, interacting with other molecular players such as the GLI 3 protein and the Hox system. Developmental independence of digit I (condensation 2) would contribute to generate a particular morphology. Macrodactyly would be linked to a variation in Hoxd-13, impacting Gli3 activity, increasing cell proliferation. The loss/reduction of digital ray/phalanges (digits II and III), would be associated to Shh activity, a mechanism inherited from the theropodan ancestry. The macrodactyly/macrodactyly coupling, and then anterior micromelia, fundamentally changed the forelimb mechanical function, compared to the 'classical' grasping structure of basal representatives and other theropods. The distal ossification of the macrodactylian digit has been identified as physiological, implying the use of the structure. However, the debate of a particular 'adaptive' use is pointless since the ecology of an organism is interactively complex, being both at the scale of the individual and dependent on circumstances. Other anatomical features also allow for compensation and a different predation (cursorial hindlimbs). This article is protected by copyright. All rights reserved.

Secreted inhibitors drive the loss of regeneration competence in Xenopus limbs

Absence of a specialized wound epidermis is hypothesized to block limb regeneration in higher vertebrates. However, the factors preventing its formation in regeneration-incompetent animals are poorly understood. To characterize the endogenous molecular and cellular regulators of specialized wound epidermis formation in Xenopus laevis tadpoles, and the loss of their regeneration competency during development, we used single-cell transcriptomics and ex vivo regenerating limb cultures. Transcriptomic analysis revealed that the specialized wound epidermis is not a novel cell state, but a re-deployment of the apical-ectodermal-ridge (AER) programme underlying limb development. Enrichment of secreted inhibitory factors, including Noggin, a morphogen expressed in developing cartilage/bone progenitor cells, are identified as key inhibitors of AER cell formation in regeneration-incompetent tadpoles. These factors can be overridden by Fgf10, which operates upstream of Noggin and blocks chondrogenesis. These results indicate that manipulation of the extracellular environment and/or chondrogenesis may provide a strategy to restore regeneration potential in higher vertebrates.

Summary: Secreted inhibitors associated with chondrogenic progression inhibit AER cell formation and restrict limb regeneration potential.

Forelimb shortening of Carcharodontosauria (Dinosauria: Theropoda): an update on evolutionary anterior micromelias in non-avian theropods

Evolutionary teratology recognises certain anatomical modifications as developmental anomalies. Within non avian-theropod dinosaurs, the strong forelimb shortening of Tyrannosauridae, Carnotaurinae and Limusaurus - associated with a reduction or loss of autonomy - have been previously diagnosed as evolutionary anterior micromelias. The feature is here examined with Acrocanthosaurus atokensis (Carcharodontosauridae) and Gualicho shinyae (Neovenatoridae). The micromelic diagnosis is confirmed for Acrocanthosaurus, without supplementary malformations. Gualicho is considered as a borderline case, outside of the micromelic spectrum, but shows a total phalangeal loss on digit III. The reduction in the biomechanical range of Acrocanthosaurus' forelimbs was compensated by the skull and jaws as main predatory organs. The same is assumed for Gualicho, but its robust first digit and raptorial claw are to be underlined. Other gigantic-sized and derived representatives of Carcharodontosauridae probably shared the anterior micromelia condition, potentially due to developmental modifications involving differential forelimbs/hindlimbs embryological growth rates, secondarily associated with post-natal growth rates leading to large and gigantic sizes; a converging state with Tyrannosauridae. Nevertheless, whereas developmental growth rates are also considered in the shortened condition of Gualicho, there is no association with post-natal gigantism. Finally, the digit III reduction likely followed the same evolutionary pathways as Tyrannosauridae, potentially involving BMPs, Fgfs and Shh signalling.

The origins, scaling and loss of tetrapod digits

Many of the great morphologists of the nineteenth century marvelled at similarities between the limbs of diverse species, and Charles Darwin noted these homologies as significant supporting evidence for descent with modification from a common ancestor. Sir Richard Owen also took great care to highlight each of the elements of the forelimb and hindlimb in a multitude of species with focused attention on the homology between the hoof of the horse and the middle digit of man. The ensuing decades brought about a convergence of palaeontology, experimental embryology and molecular biology to lend further support to the homologies of tetrapod limbs and their developmental origins. However, for all that we now understand about the conserved mechanisms of limb development and the development of gross morphological disturbances, little of what is presented in the experimental or medical literature reflects the remarkable diversity resulting from the 450 million year experiment of natural selection. An understanding of conserved and divergent limb morphologies in this new age of genomics and genome engineering promises to reveal more of the developmental potential residing in all limbs and to unravel the mechanisms of evolutionary variation in limb size and shape. In this review, we present the current state of our rapidly advancing understanding of the evolutionary origin of hands and feet and highlight what is known about the mechanisms that shape diverse limbs.This article is part of the themed issue 'Evo-devo in the genomics era, and the origins of morphological diversity'.

Xenopus Limb bud morphogenesis

Xenopus laevis, the South African clawed frog, is a well-established model organism for the study of developmental biology and regeneration due to its many advantages for both classical and molecular studies of patterning and morphogenesis. While contemporary studies of limb development tend to focus on models developed from the study of chicken and mouse embryos, there are also many classical studies of limb development in frogs. These include both fate and specification maps, that, due to their age, are perhaps not as widely known or cited as they should be. This has led to some inevitable misinterpretations- for example, it is often said that Xenopus limb buds have no apical ectodermal ridge, a morphological signalling centre located at the distal dorsal/ventral epithelial boundary and known to regulate limb bud outgrowth. These studies are valuable both from an evolutionary perspective, because amphibians diverged early from the amniote lineage, and from a developmental perspective, as amphibian limbs are capable of regeneration. Here, we describe Xenopus limb morphogenesis with reference to both classical and molecular studies, to create a clearer picture of what we know, and what is still mysterious, about this process.

Formation of a new limb bud at the boundary between a transplanted limb bud and the tail surface of Xenopus tadpoles

Through transplantation experiments with Xenopus laevis tadpoles, we found a new morphogenetic phenomenon consisting of limb bud formation at the boundary between transplanted whole limb buds and the tail surface. This phenomenon occurs without limb-limb stump interaction and has a number of unique features: (1) Only one extra limb bud was formed per transplant and the new limb and the transplanted limb were bilaterally symmetrical, forming a pair of limb girdles. (2) Extra new limb bud formation occurred not only at the tail but also at other non-limb regions, including abdominal and head surfaces. (3) Successful limb formation required the presence of basal 1/4 region (presumptive limb girdle) of a limb bud explant. (4) New limb formation was host-stage-dependent: before metamorphosis, limb bud formation ratio was high (> 90%), but as the host tadpole entered metamorphosis, this potential declined and morphological abnormalities of new limbs increased. (5) Cell lineage analysis showed that epidermis of the new limb bud always contained many (about 60%) host-derived cells, while new limb cartilage cells were completely graft-derived. These results suggest that heterotopic new limb formation occurs through interaction between graft mesenchyme and host epidermis. Thus, the present study has clarified the two important aspects of limb ontogeny: the importance of presumptive limb girdle for the limb bud initiation and the relationship between limb bud formation potential and metamorphic tissue remodeling. The present experimental system may help to improve our understanding of epithelial-mesenchymal interaction during limb bud initiation and subsequent limb cell differentiation during metamorphosis.