Comparative Aspects of Annelid Regeneration: Towards Understanding the Mechanisms of Regeneration.

The question of why animals vary in their ability to regenerate remains one of the most intriguing questions in biology. Annelids are a large and diverse phylum, many members of which are capable of extensive regeneration such as regrowth of a complete head or tail and whole-body regeneration, even from few segments. On the other hand, some representatives of both of the two major annelid clades show very limited tissue regeneration and are completely incapable of segmental regeneration. Here we review experimental and descriptive data on annelid regeneration, obtained at different levels of organization, from data on organs and tissues to intracellular and transcriptomic data. Understanding the variety of the cellular and molecular basis of regeneration in annelids can help one to address important questions about the role of stem/dedifferentiated cells and "molecular morphallaxis" in annelid regeneration as well as the evolution of regeneration in general.

Structural and Functional Characterization of the FGF Signaling Pathway in Regeneration of the Polychaete Worm Alitta virens (Annelida, Errantia).

Epimorphic regeneration of lost body segments is a widespread phenomenon across annelids. However, the molecular inducers of the cell sources for this reparative morphogenesis have not been identified. In this study, we focused on the role of fibroblast growth factor (FGF) signaling in the posterior regeneration of Alitta virens. For the first time, we showed an early activation of FGF ligands and receptor expression in an annelid regenerating after amputation. The expression patterns indicate that the entire regenerative bud is competent to FGFs, whose activity precedes the initiation of cell proliferation. The critical requirement of FGF signaling, especially at early stages, is also supported by inhibitor treatments followed by proliferation assay, demonstrating that induction of blastemal cells depends on FGFs. Our results show that FGF signaling pathway is a key player in regenerative response, while the FGF-positive wound epithelium, ventral nerve cord and some mesodermal cells around the gut could be the inducing tissues. This mechanism resembles reparative regeneration of vertebrate appendages suggesting such a response to the injury may be ancestral for all bilaterians.

Fgf- and Bmp-signaling regulate gill regeneration in Ambystoma mexicanum.

Gill regeneration has not been well studied compared to regeneration of other appendages, such as limb and tail regeneration. Here, we focused on axolotl gill regeneration and found that Fgf- and Bmp-signaling are involved in their gill regeneration mechanism. Axolotls have three pairs of gill rami, and each gill ramus has multiple gill filaments. The gills consist of mesenchyme rich in extracellular matrix and epidermis. The gill nerves are supplied from the trigeminal ganglia located in the head. Denervation resulted in no gill regeneration responses. Nerves and gills express Bmp and Fgf genes, and treating animals with Fgf- and Bmp-signaling inhibitors results in phenotypes similar to those seen in denervated gills. Inducing an accessory appendage is a standard assay in amphibian regeneration research. In our study, an accessory gill could be induced by lateral wounding, suggesting that thin axon fibers and mesenchymal Fgfs and Bmps contributed to the induction of the accessory structure. Such accessory gill induction was inhibited by the denervation. Exogenous Fgf2+Fgf8+Bmp7, which have been determined to function as a regeneration inducer in urodele amphibians, could compensate for the effects denervation has on accessory blastema formation. Our findings suggest that regeneration of appendages in axolotls is regulated by common Fgf- and Bmp-signaling cascades.

FGF and BMP derived from dorsal root ganglia regulate blastema induction in limb regeneration in Ambystoma mexicanum.

Urodele amphibians have a remarkable organ regeneration ability that is regulated by neural inputs. The identification of these neural inputs has been a challenge. Recently, Fibroblast growth factor (Fgf) and Bone morphogenic protein (Bmp) were shown to substitute for nerve functions in limb and tail regeneration in urodele amphibians. However, direct evidence of Fgf and Bmp being secreted from nerve endings and regulating regeneration has not yet been shown. Thus, it remained uncertain whether they were the nerve factors responsible for successful limb regeneration. To gather experimental evidence, the technical difficulties involved in the usage of axolotls had to be overcome. We achieved this by modifying the electroporation method. When Fgf8-AcGFP or Bmp7-AcGFP was electroporated into the axolotl dorsal root ganglia (DRG), GFP signals were detectable in the regenerating limb region. This suggested that Fgf8 and Bmp7 synthesized in neural cells in the DRG were delivered to the limbs through the long axons. Further knockdown experiments with double-stranded RNA interference resulted in impaired limb regeneration ability. These results strongly suggest that Fgf and Bmp are the major neural inputs that control the organ regeneration ability.