Regulation of Genomic Output and (Pluri)potency in Regeneration

Abstract

Regeneration is a remarkable phenomenon that has been the subject of awe and bafflement for hundreds of years. Although regeneration competence is found in highly divergent organisms throughout the animal kingdom, recent advances in tools used for molecular and genomic characterization have uncovered common genes, molecular mechanisms, and genomic features in regenerating animals. In this review we focus on what is known about how genome regulation modulates cellular potency during regeneration. We discuss this regulation in the context of complex tissue regeneration in animals, from Hydra to humans, with reference to ex vivo-cultured cell models of pluripotency when appropriate. We emphasize the importance of a detailed molecular understanding of both the mechanisms that regulate genomic output and the functional assays that assess the biological relevance of such molecular characterizations.

Keywords: chromatin; genomic; methylation; planarian; regeneration; stem cell.

Figure 1

Animal models of regeneration. (Top) Animals capable of whole-animal regeneration. Animals such as hydra and planarians have expansive regenerative capacities and constantly maintain pools of adult stem cells. (Middle) Animals capable of appendage regeneration. Animals such as the zebrafish and salamander can regenerate appendages and other complex organs. Although many of their tissues are highly similar to those of nonregenerating vertebrates, these animals respond to injury by dedifferentiating cells into lineage-restricted progenitors capable of building new complex tissues. (Bottom) Animals capable of regenerating complex tissues. Although most mammals have limited regenerative abilities, there is evidence of mammalian regeneration. As with zebrafish and salamanders, instances of mammalian regeneration often involve the formation of blastema tissue (versus scar tissue).

Figure 2

Functional tests of (pluri)potency. (a) Mammalian tetraploid complementation assay in which 2n cells are injected into a tetraploid (4n) blastocyst and allowed to develop. As the 4n blastocyst cannot form a viable embryo on its own, viable mouse embryos will develop only from 2n cells that are functionally pluripotent. (b) Planarian stem cell transplantation assay in which host worms are depleted of stem cells by a lethal dose of radiation and then injected with stem cells isolated from another worm. As lethally radiated worms will lose their ability to maintain homeostasis and regenerate, only worms rescued by injected pluripotent stem cells will recover these abilities. (c) Hydra chimera assay in which one lineage of Hydra stem cells (the interstitial cells) is ablated by treatment with the drug colchicine, leaving the animal (either a mutant or wild-type strain) with only the endodermal and ectodermal epithelial stem cell lineages. Interstitial cells from a separate animal (of the opposite strain) are then transplanted into the treated hydra; this creates a wild type + mutant chimera and reveals the contribution of a particular mutation to a cell lineage (interstitial or epithelial). Abbreviations: FACS, fluorescence activated cell sorting; WT, wild type.

Figure 3

Chromatin compaction and cellular plasticity. (a) Transmission electron micrograph of fibroblast cells in the surrounding tissue of a Mus musculus ear hole, 15 days postinjury. Image provided by and published with permission from A. Seifert. (b) Transmission electron micrograph of fibroblast cells in the surrounding tissue of an Acomys cahirinus ear hole, 15 days postinjury. Image provided by and published with permission from A. Seifert. (c) Transmission electron micrograph of a planarian neoblast cell. Image provided by and published with permission from A. Sánchez Alvarado. (d) A zebrafish retinal section stained with DAPI nuclear dye. The chromatin compaction of rod nuclei (R) appears significantly more compact and darkly stained than that of either UV cone nuclei (UV) or blue/red/green cone nuclei (elongated nuclei in top layer). Image provided by and published with permission from A. Morris. Abbreviation: DAPI, 4′,6-diamidino-2-phenylindole.