Thyrotropin-releasing hormone (TRH) promotes wound re-epithelialisation in frog and human skin

There remains a critical need for new therapeutics that promote wound healing in patients suffering from chronic skin wounds. This is, in part, due to a shortage of simple, physiologically and clinically relevant test systems for investigating candidate agents. The skin of amphibians possesses a remarkable regenerative capacity, which remains insufficiently explored for clinical purposes. Combining comparative biology with a translational medicine approach, we report the development and application of a simple ex vivo frog (Xenopus tropicalis) skin organ culture system that permits exploration of the effects of amphibian skin-derived agents on re-epithelialisation in both frog and human skin. Using this amphibian model, we identify thyrotropin-releasing hormone (TRH) as a novel stimulant of epidermal regeneration. Moving to a complementary human ex vivo wounded skin assay, we demonstrate that the effects of TRH are conserved across the amphibian-mammalian divide: TRH stimulates wound closure and formation of neo-epidermis in organ-cultured human skin, accompanied by increased keratinocyte proliferation and wound healing-associated differentiation (cytokeratin 6 expression). Thus, TRH represents a novel, clinically relevant neuroendocrine wound repair promoter that deserves further exploration. These complementary frog and human skin ex vivo assays encourage a comparative biology approach in future wound healing research so as to facilitate the rapid identification and preclinical testing of novel, evolutionarily conserved, and clinically relevant wound healing promoters

Frog serum and estrogen promote re-epithelialisation and proliferation in <i>Xenopus tropicalis</i> skin.

<p>(<b>A</b>) Representative images of re-epithelialisation sheet in control, 5% frog serum and 100 nM 17β-estradiol (Est)-treated punch wounds at day 7 in culture (scale bars: 0.5 mm). White-hatched lines indicate the leading edge of new epithelial sheets. (<b>B</b>) Representative images of PH3-positive cells in control, 5% frog serum and 100 nM 17β-estradiol-treated punch wounds at day 7 in culture. The white-hatched line demarcates new epithelial tongue. (<b>C</b>) Representative images of TUNEL-positive cells in control, 5% frog serum and 100 nM 17β-estradiol-treated punch wounds at day 7 in culture. The white-hatched line demarcates new epithelial tongue. Scale bars in (<b>B</b>) and (<b>C</b>) are 100 µm. (<b>D</b>) The graph shows the percentage reduction in wound-area of <i>Xenopus tropicalis</i> punch wounds in control, 5% frog serum (FS) and 100 nM 17β-estradiol-treated skin. (<b>E</b>) Percentage of proliferative (PH3-positive) cells present in the new epithelial tongue during re-epithelialisation. (<b>F</b>) Percentage of apoptotic (TUNEL-positive) cells present in the new epithelial tongue during re-epithelialisation. Data are mean ± SEM of 4–5 frogs (2 male and 3 female). Significance relative to control data at the same time-point denoted by *P<0.05, **P<0.01, ***P<0.001.</p

TRH increases cytokeratin 6 but not involucrin expression in human skin punch wounds.

<p>(<b>A</b>) Representative images indicated cytokeratin 6 (CK6) expression in control, 5 ng/mL and 10 ng/mL TRH-treated human skin punch wound sections, 6 Days post-wounding. The white-hatched line demarcates new epithelial tongue. (<b>B</b>) Representative images indicating involucrin (Inv) expression in control, 5 ng/mL and 10 ng/mL TRH-treated human skin punch wound sections, 6 Days post-wounding. The white-hatched line demarcates new epithelial tongue. (<b>C</b>) Quantification of involucrin immunoreactivity in the new epithelial tongues indicated in (A). (<b>D</b>) Quantification of CK6 immunoreactivity in the new epithelial tongues indicated in (B). Data are mean ± SEM of 4 female donors. Significance relative to control data at the same time-point denoted by **P<0.01, ***P<0.001.</p

Increased re-epithelialisation and proliferation in human skin following 17β-estradiol treatment.

<p>(<b>A</b>) Representative H&E stained sections of human skin punch-wounds following 6 days culture with either vehicle control, 100 nM or 1 µM 17β-estradiol (Est). New epithelial tongue indicated by red lines, scale bar represents 50 µm. (<b>B</b>) Representative images of Ki-67-TUNEL double-stained sections of human skin punch wounds following 6 days culture with either vehicle control, 100 nM or 1 µM 17β-estradiol. The white-hatched line demarcates new epithelial tongue. (<b>C</b>) The graph displays length measurements of the new epithelial tongue (as demarcated by the red lines in <b>A</b>) as analysed from H&E stained human skin sections following treatment with vehicle control, 100 nM and 1 µM 17β-estradiol. (<b>D</b>) Percentage of proliferative (Ki67-positive) cells in the new epithelial tongue of vehicle control, 100 nM and 1 µM 17β-estradiol-treated human skin wounds. (<b>E</b>) Percentage of apoptotic (TUNEL-positive) cells in the new epithelial tongue of vehicle control, 100 nM and 1 µM 17β estradiol-treated human skin wounds. Data are mean ± SEM of 4 female donors. Significance relative to control data denoted by *P<0.05, **P<0.01, ***P<0.001.</p

<i>Xenopus tropicalis</i> wound closure: promotion of re-epithelialisation and proliferation by TRH.

<p>(<b>A</b>) Representative images of the re-epithelialisation sheet in control, 10 nM and 10 µM TRH-treated punch wounds at day 7 in culture (scale bars: 0.5 mm). White-hatched lines indicate the leading edge of new epithelial sheets. (<b>B</b>) Representative images of PH3-positive cells in control, 10 nM and 10 µM TRH-treated punch wounds at day 7 in culture. The white-hatched line demarcates new epithelial tongue. (<b>C</b>) Representative images of TUNEL-positive cells in control, 10 nM and 10 µM TRH-treated punch wounds at day 7 in culture. The white-hatched line demarcates new epithelial tongue. Scale bars in (<b>B</b>) and (<b>C</b>) are 100 µm. (<b>D</b>) The graph shows the percentage reduction in wound-area of <i>X. tropicalis</i> punch wounds in control, 10 nM and 10 µM TRH-treated skin. (<b>E</b>) Percentage of proliferative (PH3-positive) cells present in the new epithelial tongue during re-epithelialisation. (<b>F</b>) Percentage of apoptotic (TUNEL-positive) cells present in the new epithelial tongue during re-epithelialisation. Data are mean ± SEM of 16 frogs (8 male and 8 female). Significance relative to control data at the same time-point denoted by *P<0.05, ***P<0.001.</p