Transcriptomic analysis of tail regeneration in the lizard Anolis carolinensis reveals activation of conserved vertebrate developmental and repair mechanisms.

Lizards, which are amniote vertebrates like humans, are able to lose and regenerate a functional tail. Understanding the molecular basis of this process would advance regenerative approaches in amniotes, including humans. We have carried out the first transcriptomic analysis of tail regeneration in a lizard, the green anole Anolis carolinensis, which revealed 326 differentially expressed genes activating multiple developmental and repair mechanisms. Specifically, genes involved in wound response, hormonal regulation, musculoskeletal development, and the Wnt and MAPK/FGF pathways were differentially expressed along the regenerating tail axis. Furthermore, we identified 2 microRNA precursor families, 22 unclassified non-coding RNAs, and 3 novel protein-coding genes significantly enriched in the regenerating tail. However, high levels of progenitor/stem cell markers were not observed in any region of the regenerating tail. Furthermore, we observed multiple tissue-type specific clusters of proliferating cells along the regenerating tail, not localized to the tail tip. These findings predict a different mechanism of regeneration in the lizard than the blastema model described in the salamander and the zebrafish, which are anamniote vertebrates. Thus, lizard tail regrowth involves the activation of conserved developmental and wound response pathways, which are potential targets for regenerative medical therapies.

A histological comparison of the original and regenerated tail in the green anole, Anolis carolinensis.

This study provides a histological comparison of the mature regenerated and original tail of the lizard Anolis carolinensis. These data will provide a framework for future studies of this emerging model organism whose genome was recently published. This study demonstrated that the cartilage skeleton of the regenerated tail enclosed a spinal cord with an ependymal core, but there was no evidence that dorsal root ganglia or peripheral nerves are regenerated. The cartilage tube contained foramina that allowed the vasculature to cross, but was otherwise a rigid structure. The original tail has muscle groups arranged in quadrants in a regular pattern that attach to the vertebral column. The regenerated tail has irregular muscle bundles of variable number that form unusual attachments to each other and to the cartilage tube. Furthermore, the data show that there was increased connective tissue within the muscle bundles. Implications for functionality of the regenerated tail and for future biomechanical studies are discussed.

The gross anatomy of the original and regenerated tail in the green anole (Anolis carolinensis).

This study investigates the gross anatomy of the original and the regenerated tail in the green anole (Anolis carolinensis). Dissections were conducted on 24 original and 13 regenerated tails. While the extrinsic muscles of the original tail in A. carolinensis are similar to those in other known Anolis lizard species, the extent of the origins of m. caudofemoralis longus and m. caudofemoralis brevis is more restricted. These differences may underlie variation in locomotor performance among anole ecomorphs. The intrinsic muscles of the original tail are also described, confirming previous findings and documenting new details, including muscle origins and insertions and the range of intraspecific variation. A comparison of the intrinsic muscles of the original tail and the regenerated tail muscles reveals key differences, such as the lack of interdigitating muscle segments and intramuscular septa in the regenerated tail. These findings, along with the replacement of interlocking vertebrae with a stiff, cartilaginous rod, suggest that important functional differences exist between the original and regenerated tail. In particular, the regenerated tail is predicted to be less capable of coordinated, fine movements. Studies of the physical properties and range of motion of the original and regenerated tail are required to test this hypothesis. This atlas of tail anatomy in A. carolinensis represents a key resource for developmental and genetic studies of tail regeneration in lizards, as well as studies of anole evolution and biomechanics.

Numb regulates Notch1, but not Notch3, during myogenesis.

In the vertebrate embryo, skeletal muscle is derived from the myotome of the somites. Notch1-3 demonstrate overlapping and distinct expression patterns in mouse somites. Notch1 and Notch2 have been shown to be inhibitors of skeletal myogenesis. The current data demonstrate that Notch3 also is an effective inhibitor of MyoD induced myogenesis. Numb, an adaptor protein that promotes Notch degradation by recruiting the E3 ubiquitin ligase, Itch, is limited in expression to dividing cells of the dorsal medial lip of the dermomyotome and the myotome itself. Here the specificity of the four protein isoforms of Numb for the Notch receptors was examined. In transcription and myogenic differentiation assays, Notch1 was consistently negatively regulated by all four Numb isoforms, and Notch3 was not a target for Numb. Notch2 however was variably affected. Subsequent analyses showed that unlike Notch1, that Notch3 was not polyubiquitinated, nor degraded when co-expressed in cells with Numb. These data provide the first observations that Notch receptors are variably affected by Numb and will be important for the interpretation of the function of Notch and Numb interactions during the development of many different cells and tissues.

Wnt3a links left-right determination with segmentation and anteroposterior axis elongation.

The alignment of the left-right (LR) body axis relative to the anteroposterior (AP) and dorsoventral (DV) axes is central to the organization of the vertebrate body plan and is controlled by the node/organizer. Somitogenesis plays a key role in embryo morphogenesis as a principal component of AP elongation. How morphogenesis is coupled to axis specification is not well understood. We demonstrate that Wnt3a is required for LR asymmetry. Wnt3a activates the Delta/Notch pathway to regulate perinodal expression of the left determinant Nodal, while simultaneously controlling the segmentation clock and the molecular oscillations of the Wnt/beta-catenin and Notch pathways. We provide evidence that Wnt3a, expressed in the primitive streak and dorsal posterior node, acts as a long-range signaling molecule, directly regulating target gene expression throughout the node and presomitic mesoderm. Wnt3a may also modulate the symmetry-breaking activity of mechanosensory cilia in the node. Thus, Wnt3a links the segmentation clock and AP axis elongation with key left-determining events, suggesting that Wnt3a is an integral component of the trunk organizer.