Why can some animals regenerate damaged tissues while others cannot? It is a question that has fascinated scientists for centuries and yet, it remains largely unanswered. From a biological perspective, we are trying to understand the molecular mechanisms that regulate and direct the regenerative response to injury and how these mechanisms are curtailed in non-regenerating systems. Patterning, growth, and cellular differentiation are integrated during regeneration and development, but it remains largely unknown if organ regeneration recapitulates embryonic development or executes its own unique program in response to injury. Using different animal models and organ systems like the skin, digit and limb we are testing how regeneration diverges from the typical scarring response in mammals. In a more clinical context, we try to apply our understanding of how tissue regeneration occurs in its natural state to develop new approaches to reduce fibrosis and scarring.
The primary focus of this research project is to understand the cellular and molecular mechanisms regulating regeneration of vertebrate skin. Despite broad understanding of mammalian skin repair processes, regenerative medicine has failed to develop therapies that can induce scar-free healing. A major goal of wound healing research is to understand the molecular events controlling fibrosis, with a view towards modulating the local injury site to promote regeneration. Traditional wound healing research, however, is heavily biased towards mammals, and a model that demonstrates scar-free healing in adults simply does not exist. At present, scarless cutaneous healing has only been described in fetal mammals, and pouch-young marsupials.
By mining the incredible regenerative ability of the axolotl (Ambystoma mexicanum), my lab has developed a model for scar-free healing of adult full thickness skin that can be directly compared to the equivalent scar outcome in mammals. The axolotl has recently emerged as a tractable genetic system to conduct regeneration and development studies. Biological resources include a genome-sequencing project (Randal Voss and Jeramiah Smith, Univ. of Kentucky), Affymetrix gene expression array (~25K genes), partial linkage map of the axolotl genome, a searchable database of protein-coding genes (www.ambystoma.org), germline transgenesis (Elly Tanaka, Max Planck Dresden, James Monaghan, Northeastern), and established GFP+ and Cherry+ transgenic lines for lineage labeling and cell isolation studies. Combining these tools with pharmacodynamics my lab is pursuing the following questions; (1) what biological components of the extracellular matrix constitute a regenerative environment, (2) how does this environment regulate a regenerative response following injury and (3) what are the key molecular signals that control fibrosis during regeneration.
Skin development and differentiation in the axolotl
A controversial hypothesis in regenerative biology asserts that regeneration is the recapitulation of embryonic development following injury. In this framework, developmental mechanisms utilized to build a structure are re-deployed to replace the missing part. In order to test this hypothesis we are currently characterizing the cellular and molecular events that occur during axolotl skin development. After identifying genetic pathways involved in epidermis, dermis and gland development and differentiation, we will be able to test if they are similarly activated during regeneration.
Understanding the similarities and differences at the cellular and genetic level between development and regeneration will shed light on whether regeneration simply activates developmental genes or actually reactivates the original developmental program. Given that key developmental genes are often activated during tumorogenesis, understanding how cell differentiation and growth are carefully regulated during regeneration may shed light ton how specific molecular pathways can control cancer progression.
Regeneration in African spiny mice (acomys)
It is generally accepted that mammals have lost much of the regenerative capacity that lower vertebrates enjoy. Alternatively, the capacity to regenerate damaged tissue may be suppressed, locked away in favor of mechanisms that promote rapid and efficient wound healing. One of my research projects sent me to Kenya where I’ve been researching the ability that two species of African spiny mice possess to “come out of their skin” when attacked by a predator. As it turns out, this is not far from the truth. Our study has found that these mice have incredibly weak skin, so weak in fact that very small amounts of tension loaded rapidly induces tearing. In response to such tearing these mice display an ability to regenerate hair follicles in the wound bed, something most mammals fail to do in response to injury. Perhaps even more exciting is the ability of these mice to heal large circular ear holes by regenerating hair follicles, dermis and cartilage. While it has been known for some time that rabbits are capable of the same fantastic feat, the wealth of resources available for work in rodents makes this a tractable system for future study. As re-emergent interest in regenerative medicine seeks to isolate molecular pathways controlling tissue regeneration in mammals, Acomys should prove useful in identifying mechanisms to promote regeneration in lieu of fibrosis and scarring.
During the regenerative process, ear tissue appears to organize into a blastema, a cellular structure that appears during epimorphic regeneration. The blastema has been best studied during salamander limb regeneration where this mass of specialized cells will eventually become the new limb. Our current work in Acomys is focused on understanding the molecular signals that generate a mammalian blastema and the specialized wound epidermis necessary to sustain regeneration. Additionally, we are focused on trying to understand how the extracellular environment may elicit a regenerative response in lieu of promoting scarring.
Does adaptive immunity constrain regenerative ability?
A longstanding question in regeneration biology centers on whether the mammalian immune system constrains regenerative capacity in terrestrial vertebrates. We can now use the natural variation we observe in Acomys to test if immune trade-offs lead to observed variation in regenerative ability within Acomys species (in terms of rate and quality) or across species (can or cannot regenerate). In collaboration with Dr. Vanessa Ezenwa (U. of Georgia) we are addressing the hypotheses that trade-offs in immunity along (1) innate vs. adaptive and (2) pro vs. anti-inflammatory axes underlie variation in the ability and rate of regeneration between these wild species. This project will take advantage of the substantial genetic resources developed for the laboratory mouse and is also part of an ongoing collaboration with Dr. Stephen Kiama at the University of Nairobi.
The effects of fundamental traits on regeneration
This project explores how fundamental traits such as body size, metamorphosis, and aging, might constrain appendage regeneration in vertebrates. As part of the Nexus Biology Group, a scientific working group committed to investigating biological systems utilizing a multidisciplinary approach, we have begun exploring the dogmatic viewpoint that bigger and older animals take longer to regenerate. In addition, we are also addressing how metamorphosis affects the trade-off between regeneration and repair. It is well known that in frogs, regenerative ability is lost at metamorphosis and we are similarly interested how this axis affects regeneration in salamanders. Our findings have uncovered that metamorphosis does effect both the rate and ability of salamanders to regenerate their limbs. This suggests that metamorphosis may constrain regeneration similarly in all amphibians. Thus anurans (frogs) may lie at one extreme along a continuum where metamorphosis ablates regenerative ability and some salamanders like newts, where metamorphosis only slightly impedes regenerative ability. The ultimate aim of this project is to determine how fundamental traits affect regenerative ability and to identify the mechanisms underlying these observations.