Homeobox-Containing Genes in Limb Regeneration

Abstract

Early investigations established an important role for homeobox-containing genes in the initiation of regeneration, as well as in the later pattern formation events leading to a new limb. The recent increased research on the mechanisms of regeneration, along with the fact that urodele amphibians provide the only opportunity to understand how vertebrates can regenerate their limbs, has led to a renewed interest in the functioning of this important group of genes during salamander limb regeneration. It appears that all vertebrates, including humans, have impressive regenerative abilities as embryos; however, all but urodeles lose much of these abilities as development proceeds. In contrast, cells in adult urodeles are unique in their ability to revert to an embryonic state (dedifferentiate) in order to recapitulate embryo-genesis, Consequently, urodeles are the only adult vertebrates that can completely and perfecdy regenerate entire limbs, and thus they offer a unique opportunity to gain critical insights for future advances in regenerative medicine. Much data indicate that a large number of homeobox genes play important roles in the initiation and regulation of limb regeneration. In some instances, the regulatory mechanisms controlling homeobox gene expression appear comparable to what is observed in developing limbs; whereas, in others they different dramatically. In spite of differences in spatial and temporal expression patterns, homeobox gene function is conserved in both regeneration and development. Research on the role of homeobox genes is poised to move forward, particularly in the context of the early stages that are unique to regeneration, and thus are critical in achieving the goal of inducing human regeneration. These efforts will be possible because of the new genetic resources for research utilizing the axolod as a model system.

Homeobox-containing genes were among the first genes identified as having a significant function in the regulation of embryonic development. Although the pioneering work was carried out with Drosophila, it soon became apparent that the structure and function of these genes is highly conserved, and that they play important roles in vertebrate development. Particularly evident was their function in the control of body and appendage pattern, thus validating the premolecular biology predictions that the mechanisms controlling pattern formation would be conserved among such divergent organisms as flies, grasshoppers and salamanders.1–3 It thus was not long before studies began to demonstrate a role for homeobox genes in the control of salamander limb regeneration.

After an exciting start, investigations into the role of homeobox genes in regeneration has languished in recent years. Fortunately, there is currently a much increased interest in regeneration, along with a renewed appreciation of the fact that urodele amphibians provide the only opportunity to understand how vertebrates can regenerate their limbs. It appears that all vertebrates, including humans, have impressive regenerative abilities as embryos; however, all but urodeles lose much of these abilities as development proceeds (see ref. 4). In contrast, cells in adult urodeles are unique in their ability to revert to an embryonic state (dedifferentiate) in order to recapitulate embryogenesis (see ref. 5). Consequently, urdeles are the only adult vertebrates that can completely and perfectly regenerate entire limbs, and thus they offer a unique opportunity to gain critical insights for future advances in regenerative medicine.

Data indicate that several homeobox genes play important roles in the initiation and regulation of limb regeneration. Our goal in writing this review is to stimulate future efforts in the field of limb regeneration research. We have elected to not consider data on the regeneration of limb buds, because they do not provide insights into the early critical events of dedifferentiation and blastema formation that are unique to adult urodeles. It is these early steps that will need to be induced in order to stimulate human regeneration. The cells of the embryo by contrast, are already immature, thus bypassing the need for dedifferentiation. Nevertheless, there is evidence for important functional roles for homeobox genes in limb bud regeneration (e.g., see refs. 6,7). In this chapter, we begin by reviewing the early studies that established the importance of homeobox genes in adult limb regeneration. In the second half, we focus on what we consider to be the emerging areas of limb regeneration research with respect to the function of homeobox genes.