Abstract
Purpose
Limb regeneration as it occurs in amphibians has two basic requirements: a source of multipotent cells capable of generating various tissues, and reorganization of those cells to form the one and only pattern of tissue appropriate to restore the missing parts. In the current biomedical world, there is much work dedicated to tissue engineering and to the differentiation of stem cells into various mature cell types. Neither of these approaches however, will by themselves succeed in regenerating a complex structure such as a limb. In our lab, we decided to focus on the pattern organization side of the equation by testing the potential of mammalian limb bud tissue to change its positional identity, and to manipulate that potential.
Method
We used mouse embryos for our mammalian model. Small groups of cells were transplanted from one region of the limb bud into another, and the resulting effect on the positional identity of those cells was assessed using molecular markers of the upper arm, forearm and hand. We knocked out a genetic regulator of cell fate named Ezh2 specifically in the limb bud to test its role in committing cells to a given positional identity along the proximodistal limb axis.
Results
We found that, similar to what one would expect for regenerating amphibians, mouse cells in the limb bud can alter their identity from that destined to make a hand to one poised to contribute to an upper arm, and vice versa. However, the cells ability to reset their identity is gradually lost during early development. In Ezh2 mutant mice, this cellular plasticity is prolonged, indicating that Ezh2 regulates commitment to a given fate. Ezh2 functions epigenetically by altering access of transcription factors to DNA.
Conclusion
There is inherent plasticity in the ability of early mammalian limb bud cells with regard to positional identity. This plasticity can be prolonged by the alteration of a single gene. Many genes related to Ezh2 are functional in regulating cell fate. Manipulation of this family of genes to facilitate pattern reorganisation among a source of stem cells capable of making every relevant tissue type is one potential approach to engineering organised tissue in mammals.
