Transcriptional changes in chick wing bud polarization induced by retinoic acid

Single-cell transcriptional landscape of temporal neutrophil response to burn wound in larval zebrafish

Neutrophils accumulate early in tissue injury. However, the cellular and functional heterogeneity of neutrophils during homeostasis and in response to tissue damage remains unclear. Here, we use larval zebrafish to understand neutrophil responses to thermal injury. Single-cell transcriptional mapping of myeloid cells during a 3-day time course in burn and control larvae revealed distinct neutrophil subsets and their cell-cell interactions with macrophages across time and conditions. The trajectory formed by three zebrafish neutrophil subsets resembles human neutrophil maturation, with varying transition patterns between conditions. Through ligand-receptor cell-cell interaction analysis, we found neutrophils communicate more in burns in a pathway and temporal manner. Finally, we identified the correlation between zebrafish myeloid signatures and human burn severity, establishing GPR84+ neutrophils as a potential marker of early innate immune response in burns. This work builds the molecular foundation and a comparative single-cell genomic framework to identify neutrophil markers of tissue damage using model organisms.

Distinct patterning responses of wing and leg neuromuscular systems to different preaxial polydactylies

The tetrapod limb has long served as a paradigm to study vertebrate pattern formation and evolutionary diversification. The distal part of the limb, the so-called autopod, is of particular interest in this regard, given the numerous modifications in both its morphology and behavioral motor output. While the underlying alterations in skeletal form have received considerable attention, much less is known about the accompanying changes in the neuromuscular system. However, modifications in the skeleton need to be properly integrated with both muscle and nerve patterns, to result in a fully functional limb. This task is further complicated by the distinct embryonic origins of the three main tissue types involved-skeleton, muscles and nerves-and, accordingly, how they are patterned and connected with one another during development. To evaluate the degree of regulative crosstalk in this complex limb patterning process, here we analyze the developing limb neuromuscular system of Silkie breed chicken. These animals display a preaxial polydactyly, due to a polymorphism in the limb regulatory region of the Sonic Hedgehog gene. Using lightsheet microscopy and 3D-reconstructions, we investigate the neuromuscular patterns of extra digits in Silkie wings and legs, and compare our results to Retinoic Acid-induced polydactylies. Contrary to previous findings, Silkie autopod muscle patterns do not adjust to alterations in the underlying skeletal topology, while nerves show partial responsiveness. We discuss the implications of tissue-specific sensitivities to global limb patterning cues for our understanding of the evolution of novel forms and functions in the distal tetrapod limb.

Development of the chick wing and leg neuromuscular systems and their plasticity in response to changes in digit numbers

The tetrapod limb has long served as a paradigm to study vertebrate pattern formation. During limb morphogenesis, a number of distinct tissue types are patterned and subsequently must be integrated to form coherent functional units. For example, the musculoskeletal apparatus of the limb requires the coordinated development of the skeletal elements, connective tissues, muscles and nerves. Here, using light-sheet microscopy and 3D-reconstructions, we concomitantly follow the developmental emergence of nerve and muscle patterns in chicken wings and legs, two appendages with highly specialized locomotor outputs. Despite a comparable flexor/extensor-arrangement of their embryonic muscles, wings and legs show a rotated innervation pattern for their three main motor nerve branches. To test the functional implications of these distinct neuromuscular topologies, we challenge their ability to adapt and connect to an experimentally altered skeletal pattern in the distal limb, the autopod. Our results show that, unlike autopod muscle groups, motor nerves are unable to fully adjust to a changed peripheral organisation, potentially constrained by their original projection routes. As the autopod has undergone substantial morphological diversifications over the course of tetrapod evolution, our results have implications for the coordinated modification of the distal limb musculoskeletal apparatus, as well as for our understanding of the varying degrees of motor functionality associated with human hand and foot malformations.

Retinoic Acid-Regulated Target Genes During Development: Integrative Genomics Analysis

Retinoic acid (RA), a major natural active metabolite of vitamin A (VA) is well known to play critical roles in embryonic development. The effects of RA are mediated by nuclear receptors (RARs), which regulate the expression of gene batteries involved in cell growth and differentiation. Since the early 1990s several laboratories have focused on understanding how RA-regulated genes and RAR binding sites operate by studying the differentiation of embryonal carcinoma cells and embryonic stem cells. The development of hybridization-based microarray technology and high performance software analysis programs has allowed the characterization of thousands of RA-regulated genes. During the two last decades, publication of the genome sequence of various organisms has allowed advances in massive parallel sequencing and bioinformatics analysis of genome-wide data sets. These new generation sequencing (NGS) technologies have revolutionized the field by providing a global integrated picture of RA-regulated gene networks and the regulatory programs involved in cell fate decisions during embryonal carcinoma and embryonic stem cells differentiation. Now the challenge is to reconstruct the RA-regulated gene networks at the single cell level during the development of specialized embryonic tissues.

RA Signaling in Limb Development and Regeneration in Different Species

This chapter brings together data on the role of retinoic acid (RA) in the embryonic development of fins in zebrafish , limbs in amphibians , chicks , and mice, and regeneration of the amphibian limb . The intention is to determine whether there is a common set of principles by which we can understand the mode of action of RA in both embryos and adults. What emerges from this synthesis is that there are indeed commonalities in the involvement of RA in processes that ventralize, posteriorize, and proximalize the developing and regenerating limb . Different axes of the limb have historically been studied independently; as for example, the embryonic development of the anteroposterior (AP) axis of the chick limb bud versus the regeneration of the limb bud proximodistal (PD) axis . But when we take a broader view, a unifying principle emerges that explains why RA administration to embryos and regenerating limbs results in the development of multiple limbs in both cases. As might be expected, different molecular pathways govern the development of different systems and model organisms, but despite these differences, the pathways involve similar RA signaling genes, such as tbx5, meis, shh, fgfs and hox genes. Studies of developing and regenerating systems have highlighted that RA acts by being synthesized in one embryonic location while acting in another one, exactly as embryonic morphogens do, although there is no evidence for the presence of an RA gradient within the limb . What also emerges is that there is a paucity of information on the involvement of RA in development of the dorsoventral (DV) axis . A molecular explanation as to how RA establishes and alters positional information in all three axes is the most important area of study for the future.

Conserved gene signalling and a derived patterning mechanism underlie the development of avian footpad scales

BACKGROUND

Vertebrates possess a diverse range of integumentary epithelial appendages, including scales, feathers and hair. These structures share extensive early developmental homology, as they mostly originate from a conserved anatomical placode. In the context of avian epithelial appendages, feathers and scutate scales are known to develop from an anatomical placode. However, our understanding of avian reticulate (footpad) scale development remains unclear.

RESULTS

Here, we demonstrate that reticulate scales develop from restricted circular domains of thickened epithelium, with localised conserved gene expression in both the epithelium and underlying mesenchyme. These domains constitute either anatomical placodes, or circular initiatory fields (comparable to the avian feather tract). Subsequent patterning of reticulate scales is consistent with reaction-diffusion (RD) simulation, whereby this primary domain subdivides into smaller secondary units, which produce individual scales. In contrast, the footpad scales of a squamate model (the bearded dragon, Pogona vitticeps) develop synchronously across the ventral footpad surface.

CONCLUSIONS

Widely conserved gene signalling underlies the initial development of avian reticulate scales. However, their subsequent patterning is distinct from the footpad scale patterning of a squamate model, and the feather and scutate scale patterning of birds. Therefore, we suggest reticulate scales are a comparatively derived epithelial appendage, patterned through a modified RD system.

Teneurins: Domain Architecture, Evolutionary Origins, and Patterns of Expression

Disruption of teneurin expression results in abnormal neural networks, but just how teneurins support the development of the central nervous system remains an area of active research. This review summarizes some of what we know about the functions of the various domains of teneurins, the possible evolution of teneurins from a bacterial toxin, and the intriguing patterns of teneurin expression. Teneurins are a family of type-2 transmembrane proteins. The N-terminal intracellular domain can be processed and localized to the nucleus, but the significance of this nuclear localization is unknown. The extracellular domain of teneurins is largely composed of tyrosine-aspartic acid repeats that fold into a hollow barrel, and the C-terminal domains of teneurins are stuffed, and least partly, into the barrel. A 6-bladed beta-propeller is found at the other end of the barrel. The same arrangement-6-bladed beta-propeller, tyrosine-aspartic acid repeat barrel, and the C-terminal domain inside the barrel-is seen in toxic proteins from bacteria, and there is evidence that teneurins may have evolved from a gene encoding a prokaryotic toxin via horizontal gene transfer into an ancestral choanoflagellate. Patterns of teneurin expression are often, but not always, complementary. In the central nervous system, where teneurins are best studied, interconnected populations of neurons often express the same teneurin. For example, in the chicken embryo neurons forming the tectofugal pathway express teneurin-1, whereas neurons forming the thalamofugal pathway express teneurin-2. In Drosophila melanogaster, Caenorhabditis elegans, zebrafish and mice, misexpression or knocking out teneurin expression leads to abnormal connections in the neural networks that normally express the relevant teneurin. Teneurins are also expressed in non-neuronal tissue during development, and in at least some regions the patterns of non-neuronal expression are also complementary. The function of teneurins outside the nervous system remains unclear.