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Inhibition of Wound Epidermis Formation via Full Skin Flap Surgery During Axolotl Limb Regeneration

Classical experiments in salamander regenerative biology over the last century have long established that the wound epidermis is a crucial signaling ...

Harvard University

Axolotls are poised to become the premiere model system for studying vertebrate appendage regeneration. However, very few molecular tools exist for studying crucial cell lineage relationships over regeneration or for robust and sustained misexpression of genetic elements to test their function. Furthermore, targeting specific cell types will be necessary to understand how regeneration of the diverse tissues within the limb is accomplished. We report that pseudotyped, replication-incompetent retroviruses can be used in axolotls to permanently express markers or genetic elements for functional study. These viruses, when modified by changing their coat protein, can infect axolotl cells only when they have been experimentally manipulated to express the receptor for that coat protein, thus allowing for the possibility of targeting specific cell types. Using viral vectors, we have found that progenitor populations for many different cell types within the blastema are present at all stages of limb regeneration, although their relative proportions change with time.

Pseudotyped retroviruses for infecting axolotl in vivo and in vitro.

Blastema formation, a hallmark of limb regeneration, requires proliferation and migration of progenitors to the amputation plane. Although blastema formation has been well described, the transcriptional programs that drive blastemal progenitors remain unknown. We transcriptionally profiled dividing and non-dividing cells in regenerating stump tissues, as well as the wound epidermis, during early axolotl limb regeneration. Our analysis revealed unique transcriptional signatures of early dividing cells and, unexpectedly, repression of several core developmental signaling pathways in early regenerating stump tissues. We further identify an immunomodulatory role for blastemal progenitors through interleukin 8 (IL-8), a highly expressed cytokine in subpopulations of early blastemal progenitors. Ectopic  expression in non-regenerating limbs induced myeloid cell recruitment, while IL-8 knockdown resulted in defective myeloid cell retention during late wound healing, delaying regeneration. Furthermore, the  receptor  was expressed in myeloid cells, and inhibition of CXCR-1/2 signaling during early stages of limb regeneration prevented regeneration. Altogether, our findings suggest that blastemal progenitors are active early mediators of immune support, and identify CXCR-1/2 signaling as an important immunomodulatory pathway during the initiation of regeneration.

Blastemal progenitors modulate immune signaling during early limb regeneration.

Formation of a specialized wound epidermis is required to initiate salamander limb regeneration. Yet little is known about the roles of the early wound epidermis during the initiation of regeneration and the mechanisms governing its development into the apical epithelial cap (AEC), a signaling structure necessary for outgrowth and patterning of the regenerate. Here, we elucidate the functions of the early wound epidermis, and further reveal  () as a dual regulator of both AEC development and inflammation during the initiation of axolotl limb regeneration. Through loss- and gain-of-function experiments, we demonstrate that  acts as both a critical survival signal to control the expansion and function of the early wound epidermis and an anti-inflammatory cytokine to resolve early injury-induced inflammation. Altogether, these findings unveil one of the first identified regulators of AEC development and provide fundamental insights into early wound epidermis function, development, and the initiation of limb regeneration.

 is a dual regulator of wound epidermis development and inflammation during the initiation of limb regeneration.

How salamanders accomplish progenitor cell proliferation while faithfully maintaining genomic integrity and regenerative potential remains elusive. Here we found an innate DNA damage response mechanism that is evident during blastema proliferation (early- to late-bud) and studied its role during tissue regeneration by ablating the function of one of its components, Eyes absent 2. In  mutant axolotls, we found that DNA damage signaling through the H2AX histone variant was deregulated, especially within the proliferating progenitors during limb regeneration. Ultimately, cell cycle progression was impaired at the G1/S and G2/M transitions and regeneration rate was reduced. Similar data were acquired using acute pharmacological inhibition of the Eya2 phosphatase activity and the DNA damage checkpoint kinases Chk1 and Chk2 in wild-type axolotls. Together, our data indicate that highly-regenerative animals employ a robust DNA damage response pathway which involves regulation of H2AX phosphorylation via Eya2 to facilitate proper cell cycle progression upon injury.

Stephanie Tsai

Department of Stem Cell and Regenerative Biology,
Department of Molecular and Cellular Biology

Harvard University

Inhibition of Wound Epidermis Formation via Full Skin Flap Surgery During Axolotl Limb Regeneration

Stephanie Tsai

.css-o250i3{font-size:18px;font-family:Open Sans,sans-serif;line-height:21.6px;}Department of Stem Cell and Regenerative Biology,Department of Molecular and Cellular Biology

Harvard University

Inhibition of Wound Epidermis Formation via Full Skin Flap Surgery During Axolotl Limb Regeneration

Department of Stem Cell and Regenerative Biology,
Department of Molecular and Cellular Biology