Extracellular Caspase-1 induces hair stem cell migration in wounded and inflamed skin conditions

The wound-healing process is a paradigm of the directed migration of various pools of stem cells from their niche to the site of injury where they replenish damaged cells. Two decades have elapsed since the observation that wounding activates multipotent hair follicle stem cells to infiltrate the epidermis, but the cues that coax these cells out of their niche remain unknown. Here, we report that Caspase-1, a protein classically known as an integral component of the cytosolic inflammasome, is secreted upon wounding and has a non-canonical role in the extracellular milieu. Through its caspase activation recruitment domain (CARD), Caspase-1 is sufficient to initiate the migration of hair follicle stem cells into the epidermis. Uncovering this novel function of Caspase-1 also facilitates a deeper understanding of the mechanistic basis of the epithelial hyperplasia found to accompany numerous inflammatory skin diseases.