Hair follicles cyclically degenerate and regenerate through adult life: after an initial growth phase, hair follicles enter a destructive phase and then go through a quiescent stage before re-entering the next growth phase. This cycling involves hair follicle stem cells (HFSCs) but how these cells transition between the phases of the hair follicle cycle is unclear. Here, Hoang Nguyen and colleagues report that the forkhead transcription factor Foxp1 is crucial for maintaining HFSC quiescence (). The authors show that Foxp1 is expressed in adult mouse HFSCs and that ablation of Foxp1 in skin epithelial cells shortens the quiescent phase of the hair cycle and causes precocious HFSC activation. Furthermore, they report that overexpression of Foxp1 in keratinocytes leads to cell cycle arrest as well as to upregulation of Fgf18, which has been previously implicated in controlling HFSC quiescence. Finally, the researchers demonstrate that exogenously delivered FGF18 can prevent the HFSCs of Foxp1-null mice from being prematurely activated, confirming that FGF18 acts downstream of Foxp1 to regulate stem cell quiescence.
