As the steadfast protector of the body's interior, the skin must be able to swiftly respond to stressors from the outside world and then return to a steady state in order to maintain everyday operations. Coordinating that agile response are epidermal stem cells, which produce the overlying protective layers of specialized skin cells sent to the surface while also maintaining a ready pool of new stem cells to draw from.
The mechanisms that regulate this system to constantly maintain a normal flux have long remained elusive. Now researchers at Rockefeller University have discovered a regulatory circuit connecting the skin surface to stem cells via Langerhans immune cells, whose cellular projections span the epithelial layers and transmit signals between the two, that acts as a system regulator. They published their results in Science.
"This circuit acts like a rheostat that continuously fine tunes the stem cells' response to the environment at the skin surface," says first author Anita Gola, a research associate in the Robin Chemers Neustein Laboratory of Mammalian Cell Biology and Development , led by Elaine Fuchs . "Langerhans cells pick up signals from microbes on the skin surface and, through chemical messaging, signal to stem cells whether they need to produce new stem cells or direct cells to differentiate."
"Essentially, Langerhans cells act as liaisons to regulate host-microbe interactions, while joining forces with the stem cells to maintain the integrity and antimicrobial peptide production of the skin's barrier," Fuchs says.
Seeking the system
Fuchs's lab has already discovered epidermal circuits that operate in response an acute inflammation and wound repair, and that allow the skin to mount and retain epigenetic memories of its past experiences. Here, they set out to find another unknown mechanism: the communication system linking epidermal stem cells to signals coming from microbes on the body surface, which enables dynamic responses to ever-changing environmental factors.
They suspected the signaling network involved nearby cells in the local "niche"—a kind of cellular and physical neighborhood—and within this niche in mouse skin there were only two potential candidates: dendritic epithelial T cells (DETCs), a type of specialized lymphocyte, and Langerhans cells (LCs), a classic antigen-presenting cell, known to be a central player in driving adaptive immune responses in lymph nodes. Much of this view traces back to Rockefeller's Ralph Steinman, whose pioneering work established LCs as members of the dendritic cell family; Steinman was awarded the 2011 Nobel Prize for his discovery of dendritic cells and their role in adaptive immunity.
They leaned towards LCs, which concentrate in vulnerable barrier areas such as the skin, mouth, and nose. "We know that macrophages in general are highly sensitive to their local environment and often have embedded functions that maintain their niches, so we wondered whether Langerhans cells, which spend their lives in sustained contact with epidermal stem cells and can extend their dendrites right through the skin's epithelial layers, might have additional, stem cell regulatory roles beyond their well-appreciated immunological ones," says Gola.
Keeping stem cells in check
Using genetic LC knockout approaches in a series of in vivo experiments, the Fuchs lab found that animals lacking LCs showed increased epidermal stem-cell proliferation and differentiation. "This suggested that the stem cell compartment wasn't behaving exactly as it should in a homeostatic condition," Gola says. "That was our first smoking gun."
By probing gene expression in these cells, they found a gene called Ptgs1 was uniquely enriched in Langerhans cells when compared to all other skin resident immune cells. The gene codes for the enzyme COX-1, which is essential for the production of prostaglandins, a class of hormone-like lipid mediators involved in a multitude of cellular functions. In Langerhans cells, the high expression of COX-1 led to the robust production of a type of prostaglandin called PGE2. They then discovered that epidermal stem cells have only one prostaglandin receptor— EP4, and it's the one specific for PGE2.
Further experiments revealed what happens when there isn't enough PGE2 for the stem cells: "Without this brake on proliferation, stem cell proliferation increases," says Fuchs. "Their progeny don't completely finish differentiating before reaching the skin surface, which means that they can't perform quite as well the tasks they're supposed to do. It's like sending in a team of amateurs to do work that requires professional expertise."
That incomplete development led to specific problems. "We found that immature epithelial cells had extreme responses to benign bacteria that would normally reside on mouse skin without issue," Gola notes. "We also saw structural weakening of the skin barrier, an overgrowth of different types of native bacteria, and a decrease in the amount of antimicrobial peptides that the skin should make to maintain a healthy microbiota."
The presence of Langerhans cells prevented these negative outcomes. "They're attuned to signals from microbiota at the surface and adjust production of PGE2 and the levels of COX-1 enzyme in response to those signals, which puts the brakes on epidermal stem cell proliferation by suppressing a signaling pathway associated with cell division," she says. "Simultaneously, they promote the production of differentiated cells."
"We think that there is some redundancy in the system, so it could be that this COX-1 prostaglandin regulator integrates multiple signals upstream," Gola says.
Pathogens and inflammation
"Anita did such a beautiful job of sleuthing out this circuit step by step and demonstrating that the circuit is constantly being fine tuned to respond to an ever-changing suite of stressors," Fuchs says. "Even something as mundane as washing your hands may require this type of fine-tuned regulation to help the skin barrier maintain homeostasis."
The researchers theorize that because the cycle of response and reset is so essential to protecting the skin barrier, this system may integrate a number of signals to maintain local skin fitness. Looking forward, the Fuchs lab is excited to now explore how Langerhans cells respond to negative inputs such as pathogenic bacteria and inflammation.
