Regeneration

Hydra, like a number of other organisms, has the ability to regenerate. It
is one of the most commonly studied of the regenerating organisms.

(Galliot and Chera)

There are many different phyla of animal that contain species with a high potential for regeneration.

Why Study Regeneration?

Though adult humans cannot regenerate entire lost body parts, they do have limited cell renewal; for example, repairing a cut in the skin.
Study of regeneration in other organisms can aid in current research on how to regenerate human organs for patients with a damaged/dysfunctional organ.

Why Use Hydra?

Hydra are far less complex than vertebrates.
Other common invertebrate model organisms (nematodes and Drosophila) are not capable of regeneration as adults.

Overview of Regeneration

All body column cells are continuously undergoing mitosis. They migrate to the extremities where they are eventually shed. Therefore, the Hydra is constantly regenerating.

There is higher cell cycling activity in the body column of the Hydra. Brighter green indicates more cell cycling. Notice that there is minimal green coloration of tentacles.

If the body is cut in half, the lower half will grow a new head, and the upper half will grow a new foot. If the head and foot are both cut off, a new head and foot will grow, and the Hydra will maintain its original polarity.

The top half grows a new foot, and the bottom half grows a new head.

If a Hydra is put in a blender, then the cells are reaggreated in a centrifuge, the Hydra will reform its original structure.

A Hydra reforms after being blended then centrifuged.

Polarilty of the Hydra is regulated by the head activation gradient (highest concentration at head) and foot activation gradient (highest concentration at foot).
The method of head regeneration varies depending on how far down the body axis the amputation occurs.
Hydra head regeneration relies on activation of the Wnt-beta-catenin pathway.

Hydra Wnt Genes (Lengfeld et al.)

There are 11 Wnt genes in Hydra, including HyWnt3.
Cysteine residue patterns confirmed that the identified genes are Wnts. They are compared to other, previously identified Wnt orthologs.

Cysteine residue patterns in HyWnt7 and its orthologs are similar.

All 11 Hydra Wnt genes are homologs of bilaterian Wnt genes.
Phylogenetic analysis was used to determine that 8 of the 11 are orthologs to mammalian genes (the other 3 are also closely related to mammalian genes).
The genomic structure of the Wnt genes was compared to human genes. It was found that Wnt gene subfamilies are highly conserved from Hydra to humans.

Wnt Gene Expression

There are 7 Wnt genes expressed in the adult Hydra hypostome
(distal end of body column).
They differ in where they are expressed within the hypostome: Expression ranges from only in the apical tip to throughout the hypostome.

Expression of HyWnt3 is limited to the apical tip of the hypostome.

HyWnt7 is expressed in most of the hypostome.

There is expression in both endoderm and ectoderm layers of epithelial cells, but 6 out of the 7 are more highly expressed in the endoderm.

The red arrows point to the boundary between endoderm and ectoderm. The darker blue below the line indicates that there is higher expression in the endoderm.

There are 2 Wnt genes (HyWnt5 and HyWnt8) that are expressed in the tentacles around the hypostome.

Expression of HyWnt3

The gene HyWnt3 is expressed earlier than any other Wnt gene during head regeneration.
This indicates that HyWnt3 may be a “master” Wnt ligand that stimulates other signaling pathways.

A mutant strain of Hydra, reg-16, was used to study the role of HyWnt3.
Reg-16 Hydra have impaired head regeneration and a lack of HyWnt3 expression.
When reg-16 Hydra were decapitated, 30% of them were successful in regenerating a new head.
However, when reg-16 Hydra were decapitated then incubated with recombinant HyWnt3, 70% of them grew new heads.
A similar result occurred when HyWnt3 was replaced with the mouse gene Wnt3a in reg-16 Hydra.
Lastly, if a decapitated reg-16 Hydra was treated with alsterpaullone, which stimulates canonical Wnt signaling, 90% regenerated a head.

Grey bars are the controls, red bars are the experimental replicates. The * indicates that the results are significant.

HyWnt3 also plays an important role in setting up the head organizer, which is responsible for axial patterning.

Areas not addressed by the Lengfeld paper:

Incubation of reg-16 Hydra in HyWnt3 only results in 70% head regeneration. So which other genes are mutated in reg-16 Hydra?
If HyWnt3 is the “master” Wnt ligand, what is the mechanism for how it triggers other signaling pathways?

Hydra Wnt Signaling Conclusion:

Studying Wnt signaling in Hydra could provide valuable insight into the formation of the primary body axis of the early metazoans. Some components of Wnt signaling have been largely conserved throughout evolution of animals. It is important to learn more about the Wnt signaling pathway because many different diseases are caused by mutations in Wnt signaling.

Other Resources

Wnt Signaling: The Wnt Homepage

Regenerative Medicine: Wake Forest Institute for Regenerative Medicine

Rengenerative Research At GT/Emory: The Davis Group, Sambanis Laboratory

References

Galliot, Brigitte, Simona Chera. “The Hydra model: disclosing an apoptosis-driven generator of Wnt-based regeneration.” Trends in Cell Biology. Vol.20 No.9 (2010) 514-523. (URL link)

Lengfeld, Tobias, Hiroshi Watanabe, Oleg Simakov, Dirk Lindgens, Lydia Gee, Lee Law, Heiko A. Schmidt, Suat Özbek, Hans Bode, Thomas W. Holstein. “Multiple Wnts are involved in Hydra organizer formation and regeneration.” Developmental Biology. 330 (2009) 186–199. (URL link)