Skin Development Finding Could Lead to Novel Therapies for Burns and Birth Defects

University of Colorado Boulder researchers have discovered a key mechanism by which skin begins to develop in embryos, shedding light on the genetic roots of birth defects like cleft palate and paving the way for development of more functional skin grafts for burn victims.

“This study maps how skin development starts, from the earliest stages,” says Rui Yi, Ph.D., associate professor of molecular, cellular, and developmental biology, at the University of Colorado Boulder and senior author of the paper (“Single Cell and Open Chromatin Analysis Reveals Molecular Origin of Epidermal Cells of the Skin”) published in Developmental Cell.

“How embryonic progenitors coordinate cell fate specification and establish transcriptional and signaling competence is a fundamental question in developmental biology. Here, we show that transcription factor ΔNp63 profoundly changes the transcriptome and remodels thousands of open chromatinregions of Krt8+ progenitors during epidermal fate specification. ATAC-seq and single-cell RNA-seq reveal that ΔNp63-dependent programs govern epidermal lineage formation, and ΔNp63-independent programs, mediated by AP2 and AP1 transcription factors, promote epidermal differentiation and epithelial-to-mesenchymal transition. ΔNp63 promotes Wnt signaling by directly upregulating Wnt ligands, Frizzled receptors, and transcription factors. Deletion of β-catenin in Krt8+ progenitors delays their maturation into Krt5+ progenitors,” write the investigators.

“The lack of epidermal Wnt production in the absence of ΔNp63 also incapacitates Wnt activation in the underlying dermal cells. These findings reveal the remarkable changes of the transcriptome, open chromatin, and signaling pathways at the onset of skin development and uncover the molecular cascade for epidermal lineage formation.”

Thousands of people undergo skin grafts each year to repair burns, birth defects, or wounds. Medical advancements, including the advent of stem cell therapy which uses the patient’s own skin cells to grow new skin, have improved skin transplants. But replacement skin often lacks important features like hair follicles, sweat glands, or nerve endings.

“Skin is an incredibly complex system and the regeneration we are doing now is not even close to duplicating it,” says Dr. Yi. “The overarching goal is to someday be able to regenerate fully functional skin, and to do that, we have to know, fundamentally, what happens at the beginning.”

For the study, Dr. Yi’s team used genomic tools and DNA sequencing techniques to observe what happens inside embryonic progenitor cells of mice as they coordinate to form skin. The research focused on transcription factor p63, found mostly in skin cells and long-known to play a critical role in skin formation. Previous studies have shown that mice born without p63 have no skin and malformed limbs. Humans with p63 mutations often have cleft lips or other malformations of the teeth and skin. In adults, loss of p63 function is associated with metastatic cancer.

“We have known for a long time that this transcription factor is probably the most important for skin development. What we have not known is what it does,” says Dr. Yi.

Using fluorescent tags that illuminated cells where p63 was present, and RNA sequencing to examine patterns of gene expression, the researchers examined cells from day 9 to 13 of a 19-day mouse gestation, the time when skin is believed to form.

They found that P63 was responsible for switching on at least 520 genes and igniting numerous critical signaling pathways, including the Wnt pathway (responsible for hair follicle formation), the Eda pathway (critical for the formation of hair follicle, sweat glands and teeth) and the Notch pathway (responsible for prompting stem cells to differentiate into the epidermis.)

They also found that this process began earlier than previously believed and impacted thousands of regions of the genome that govern skin and limb formation.

“Our study provides mechanistic insights into the critical role of p63 at the onset of skin development and reveals a molecular basis for explaining how p63 mutations in humans can cause so many skin diseases,” explains Dr. Yi, adding that the study was in mice and further studies using human cells are needed.

But, if replicated, the findings could help researchers develop new prenatal tests and treatments for skin-related birth defects. The research could also inform the development of methods to coax adult cells to behave more like embryonic ones and generate fully functional skin.

“Instead of just grafting a piece of skin to cover your body, you could regenerate it as if it were going through development for the first time,” notes Dr. Yi.