| Abstract |
Regeneration is the restoration of a body part after injury or as a normal bodily
process. Many animals are capable of regenerating different body parts. Salamanders
regenerate their tail and legs, crabs regenerate their appendages, flatworms can
regenerate their entire body, humans can regenerate part of their liver, etc. The
phylogenetic distribution of regenerative capacity in the animals is still unresolved.
There are closely related species with very different regenerative capacities. To
understand how regeneration is achieved and how regenerative capacity evolved in
the Metazoa, we need to collect information from diverse species.
Traditionally studied model organisms, such as flies, C. elegans and mice have poor
regenerative capacities. On the other hand, animals with extended regenerative
capabilities, such as starfish, crabs, flatworms and salamanders, are not amenable to
genetic manipulation. Parhyale hawaiensis is capable of regenerating all of its
appendages within approximately one week after amputation. Moreover, work from
our and other labs over the last ten years has developed a series of tools, rendering
Parhyale an attractive model for regeneration studies.
The main goal of my PhD thesis was to establish Parhyale hawaiensis as the first
arthropod regeneration model organism. To achieve this, I described limb
regeneration in Parhyale, I studied the level of commitment of progenitor cells that
generate the new tissue and identified a cell type that participates in muscle
regeneration in Parhyale. I compared my findings with what is known in other
regeneration models to gain information about the evolutionary history of tissue
regeneration.
Initially, I identified that Parhyale is able to regenerate all of its appendages within 5-
8 days, restoring all of the appendages’ cell types, such as epidermis, neurons and
muscles. I established a timeline of the process of appendage regeneration in
Parhyale, identifying five distinct stages. (a) Wound closure, (b) blastema formation,
(c) patterning, (d) growth and (e) muscle regeneration.
6
Animals that are capable of regeneration employ two very different strategies.
Planarians use totipotent cells to regenerate their missing parts. On the other hand,
vertebrates activate committed progenitors to regenerate specific tissues. To examine
the level of commitment of the progenitors during Parhyale limb regeneration, I
generated mosaic animals expressing a fluorescent protein under the control of the
heat-shock promoter in specific lineages. I subjected these animals to amputation and
assessed the participation of the labeled cells in regeneration. I identified that the
progenitors are lineally restricted and reside locally, indicating the absence of a single
pool of progenitors. These experiments clearly indicate that Parhyale resembles
vertebrates in that it uses committed rather than totipotent progenitors.
I also identified a Pax3/7-expressing mesodermal cell type closely attached to the
muscle fibers. These cells were highly reminiscent of the muscle satellite cells that
have been described in vertebrates; therefore, I named them satellite-like cells (SLCs).
Homologs of satellite cells had never been identified earlier outside the chordates.
Satellite cells have been described in vertebrates to participate in muscle repair,
growth and homeostasis. They are molecularly characterized by the expression of the
Pax3/7 family of transcription factors. Their participation in limb, fin and tail
regeneration differs among species. By Edu staining and transplantation experiments,
I was able to show that SLCs participate in the formation of the blastema, as well as
in muscle formation during appendage regeneration.
Collectively, these results highlight a number of key similarities in regeneration
between arthropods and vertebrates, arguing for common cellular mechanisms that
were present in their last common ancestor. Moreover, the discovery of SLCs in
arthropods pushes back the origin of satellite cells to the last common ancestor of
protostomes and deuterostomes, at the base of the bilaterians. This could imply that
the ability to regenerate muscles has a common origin dating back to precambrian
times. Alternatively, muscle regeneration may have evolved independently in
arthropods and vertebrates, engaging a homologous cell type – satellite cells – as the
source of regenerated muscles.
|
Appendage regeneration in the amphipod crustacean parhyale hawaiensis
