A Tale of Four Studies: Valentina Greco’s Lab Uncovers the Fascinating Dynamics of Skin Regeneration

This spring brought joyful news for NYSCF – Robertson Stem Cell Investigator Alumna Valentina Greco, PhD, and her laboratory at Yale University, as four of their studies were accepted for publication in a matter of a few weeks: the scientific equivalent of finishing four marathons.

It is the culmination of their years of work, some of which during a pandemic, to decipher some of the biggest mysteries behind the largest organ in our bodies: the skin. These studies shed light into how the skin develops during early life, how it shields our bodies from external threats, and surprising revelations about how it neutralizes early stages of cancer.

Live Imaging Gets Under the Skin

The skin has a remarkable regenerative capacity, renewing itself every 4-8 weeks to cope with the continuous exposure to foreign threats. This demanding regenerative process is orchestrated by the stem cells that live there, whose function is to provide a pool of dividing cells to replenish old or damaged ones.

To track skin stem cells in their native environment over time and examine their activity, the Greco lab has used advanced imaging in living mice (also called live imaging) for over a decade, largely inspired by work of many colleagues in the field of developmental biology.

In two papers, the team took this well-developed technique to the next level, combining it with molecular and computational tools to reveal how stem cells communicate with one another, and even how they change the way in which their DNA is compacted as they turn into more specialized cells, an important process for maintaining skin.

In the first study, published in the Journal of Cell Biology, the team combined live imaging of the mouse epidermis (the outermost layer of the skin) with machine learning to show that stem cells located in different areas of the skin launch coordinated calcium signals across remarkably long distances to prompt the replacement of lost cells.

“Before we only understood how calcium – an important cellular messenger – spreads from one skin cell to its direct neighbor in vivo. Here we’ve been able to look across more than 3,000 cells and found that there is a broader, tissue-wide coordination of those signals during skin regeneration,” explains Jessica Moore, PhD, first author of the study.

Video caption: Skin stem cells (labeled in magenta) launch coordinated calcium signals (labeled in green), which dictate where and when lost cells are replaced. Credit: the Greco lab.

Notably, this observation was inspired by the work of neuroscientists.

“We adapted the calcium sensor that is often used for live imaging of the brain to look at the skin of a mouse,” continues Dr. Moore. “I was taking images of the mouse ear every two seconds for 30 minutes so that we could capture these fast calcium signals across thousands of cells.”

In their second study, spearheaded by Dennis May, PhD, and Sangwon Yun, PhD, and co-authored by NYSCF – Druckenmiller Fellow Alumni Katie Cockburn, PhD, and Sangbum Park, PhD, the Greco lab leveraged an existing tool to label DNA and coupled it with live imaging to examine the DNA architecture of skin stem cells as they undergo the process of specialization, in real-time and at a higher resolution than previously possible.

Scientists knew that DNA architecture is remodeled when a stem cell turns into a more specialized cell, but they did not understand how and when this happens relative to all the other changes in the cell (e.g., in its shape and behavior).

“We found that there are gradual changes in the DNA packaging of stem cells before they assume a specialized role, and that this is associated with the activation of several key genes,” explains Dr. Greco. The research appears in eLife.

This is important because it transforms the way we think about transitions from one cell type to another.

“We tend to think about cellular transitions as switches, because we like to simplify problems by being binary – like when we want to simulate the transition between day and night by turning the lights on and off – but in reality, most changes happen gradually,” reflects Dr. Greco.

Healthy Skin Cells Square Up Against Cancer

 Another research aim in the Greco lab is to understand how skin is maintained in the face of injury and mutations. As we age and are exposed to environmental insults, a growing number of skin cells accumulate genetic mutations, including those that can cause cancer, making our skin a mosaic of healthy and mutation-bearing cells.

Scientists have long believed that simple wounds or surgery increase the risk of skin cancer by increasing the number of mutated cells, but the team’s study challenges this notion. Surprisingly, they found that in mosaic skin, injury promotes the expansion of healthy cells, which in turn suppress the growth of mutated cells. The work is published in Nature.

“Our findings completely change the way we think about cancer initiation, and they suggest that acute injury may actually prevent rather than promote tumor formation,” says Sara Gallini, PhD, first author of the study.

This work could represent a paradigm shift in how we think about cancer treatment.

“This research encourages us to move away from eliminating the mutant cells, which is the current focus of anti-cancer therapies, towards empowering our body’s natural capacity to combat tumor formation, and eventually widen therapeutic options against cancer,” continues Dr. Gallini.

The team now plans to look at this phenomenon during aging, and investigate whether the capacity of healthy cells to neutralize the mutant ones is impaired as we grow old.

A New Player in the Skin Cell Repertoire

While we tend to think most about the epidermis, the outer layer of the skin which functions as a physical barrier to the external environment, the skin is actually a large organ with many different components. The Greco lab was intrigued to look into a historically understudied one: its blood vessels. It is well known that a functional vasculature is essential for organ growth and maintenance, yet how blood vessels develop and are established within the skin remained unclear.

In a new study in Cell, the lab used live imaging in mice to follow the cells that line the inside of blood vessels (known as endothelial cells) from birth all the way into adulthood. They found that, in newborn mice, endothelial cells rearrange their position within the vasculature, eliminating connections between some vessels in favor of reinforcing the ones needed for tissue growth.

Remarkably, they found that this migratory behavior stopped in adult mice, but could be reactivated in response to injury to allow for vessel repair.

“Completely surprisingly, we discovered that endothelial cells in adult mice are capable of self-repair, resealing their cell membranes following injury. This was already a known property of muscle tissue, but it was not known that endothelial cells had evolved similar abilities in vivo,” notes Chen Yuan Kam, PhD, first author of the study.

Altogether, these studies paint a more detailed picture of how our skin develops and functions, and hint at new ways we can leverage these processes to find better treatments.

While reflecting on the discoveries, Dr. Greco highlights the contributions from both local and international collaborators who brought unique insights and capabilities to this work, including Drs. Smita Krishnaswamy and Lauren E. Gonzalez from Yale University, Dr. Maria Kasper from Karolinska Institute, and Dr. Karen K. Hirschi from University of Virginia.

“In addition to the incredible talent that I am lucky to partner with in my lab, our collaborators have been instrumental to expand our expertise and understanding, compensating our deficiencies.”

Congratulations to Dr. Greco and team on their exciting new findings!