Insights into regeneration tool box: An animal model approach

Abstract

For ages, regeneration has intrigued countless biologists, clinicians, and biomedical engineers. In recent years, significant progress made in identification and characterization of a regeneration tool kit has helped the scientific community to understand the mechanism(s) involved in regeneration across animal kingdom. These mechanistic insights revealed that evolutionarily conserved pathways like Wnt, Notch, Hedgehog, BMP, and JAK/STAT are involved in regeneration. Furthermore, advancement in high throughput screening approaches like transcriptomic analysis followed by proteomic validations have discovered many novel genes, and regeneration specific enhancers that are specific to highly regenerative species like Hydra, Planaria, Newts, and Zebrafish. Since genetic machinery is highly conserved across the animal kingdom, it is possible to engineer these genes and regeneration specific enhancers in species with limited regeneration properties like Drosophila, and mammals. Since these models are highly versatile and genetically tractable, cross-species comparative studies can generate mechanistic insights in regeneration for animals with long gestation periods e.g. Newts. In addition, it will allow extrapolation of regenerative capabilities from highly regenerative species to animals with low regeneration potential, e.g. mammals. In future, these studies, along with advancement in tissue engineering applications, can have strong implications in the field of regenerative medicine and stem cell biology.

Keywords: Epimorphosis; Evolutionarily conserved pathways; Morphallaxis; Novel genes; Regeneration; Regulation; Tissue engineering; Wnt.

Figure 1.. Modes and different classes of regeneration.

Regeneration can occur by either one or combination of these three modes (1) Rearrangement of pre-existing tissue, (2) Use of adult somatic stem cells (3) dedifferentiation and/or transdifferentiation of cells. Regeneration is classified into five different types. In vertebrates, the regeneration response is activated by both stem cell proliferation, and dedifferentiation or transdifferentiation of the cells present adjacent to the amputated stump. The amputated stump responds to the stimulus to regenerate and eventually cells nearby undergo determination, differentiation, and scale up to the appropriate polarity to replace the missing tissue in same proportion.

Figure 2.. Mechanism of regeneration.

(A) The basic mechanism of how regeneration takes place. (B) In invertebrates (Planaria, and Hydra) whole body regeneration takes place, while as in (C) vertebrates (Newt limb, and Zebrafish fin regeneration) structural regeneration takes place (Sanchez Alvarado and Tsonis, 2006).

Figure 3.. Regeneration response is non-uniformly distributed throughout the animal kingdom- Phylogenetic distribution and corresponding model species.

(A) The taxa which contains at least one species that are capable of regeneration are shown in colored background. The taxa that show whole body regeneration are shown in green. The taxa that show structural regeneration are shown in Blue, and the taxa that show both whole body as well as structural regeneration are shown in Cyan. For the remaining taxa where regeneration has not been reported or the species where its presence is unknown. (B) The diverse animal phylogeny showing regeneration include animal models like, Hydra, Planaria, Newts, and Zebrafish (Classic animal models of regeneration) (Sanchez Alvarado and Tsonis, 2006).

Figure 4.. Regeneration in fresh water polyp, Hydra (Hydra vulgaris).

(A) Nomenclature used to describe different body parts of Hydra anatomy. Head (Hypostome) region is decorated by tentacles surrounding primitive mouth, a body column serves as gastric cavity, peduncle is the lower quarter of the body column that stores most of gastro vascular fluid and pumps it into rest of the cavity (Shimizu and Fujisawa, 2003), foot is used to adhere to the substrate, bud is used to accomplish asexual reproduction. Hydra regeneration is accomplished by three different stem cell populations: endoderm epithelia, ectoderm epithelia, and interstitial stem cells. (B-D) Role of Wnt signaling pathway during three different types of regeneration that can be studied in Hydra (B) Apical head regeneration (C) Basal head regeneration (D) Foot regeneration.

Figure 5.. Fresh water Planaria, Schmidtea mediterranea, exhibits regeneration response.

(A) Nomenclature used to describe different body parts of planarian anatomy, (B) Types of amputations to study regeneration response in Planaria, (C) Normal regeneration response in Planaria. Wound epithelium formed after amputation sends signal that promote stem cell proliferation and regeneration response. (D and E) Perturbed regeneration response (D) β-catenin RNAi treated Planaria induce posterior blastema to regenerate into head. (E) RNAi of APC, a negative regulator of Wnt-β-catenin signaling pathway, in Planaria induce anterior blastema to regenerate into tail.

Figure 6.. Regeneration mechanism in Newts.

(A) Notopthalmus viridescens exhibits strong regeneration potential, (B) Limb regeneration in Newt is a classic example of normal (epimorphic) regeneration response. The cell at the site of amputation promote blastema formation without drastic rearrangement of remaining tissue. Blastema initiates regeneration response. (C, D) The cross talk between evolutionary conserved pathways, Hedgehog (Hh) and Wnt regulate limb regeneration in newts.

Figure 7.. Regeneration response of Zebrafish (Danio rerio), a teleost (bony fish).

(A) Zebrafish model exhibits regeneration potential, (B) In Zebrafish, Wnt- β-catenin pathway gets upregulated during fin regeneration, (C) Wnt- β-catenin pathway when inhibited perturbs blastema formation, and blocks fin regeneration.

Figure 8.. Regeneration response in Drosophila melanogaster model system.

(A) Drosophila larval imaginal discs exhibit regeneration. The imaginal disc, and the adult structure that they regenerate, are shown in corresponding colors. (B) Effect of Wingless (Wg, ortholog of vertebrate Wnt1 in Drosophila) on wing regeneration (early vs late developmental stage) in Drosophila. Wg follows the canonical β-catenin signaling pathway.

Figure 9.. Transdetermination response in Drosophila.

Certain transdetermination events are more probable than others (Worley et al., 2012). The thickness of the arrow is used to indicate the relative likelihood of the event.