FAP-intrinsic Hedgehog signaling controls intramuscular adipogenesis, fibrosis, and myofiber regeneration

Fibro/adipogenic progenitors (FAPs) are multipotent stromal cells that support myofiber regeneration, but can also give rise to intramuscular adipose tissue (IMAT) and fibrotic scar tissue. While the Hedgehog pathway suppresses FAP adipogenesis and promotes myofiber repair through ligand Desert Hedgehog, the key cell type that senses this signal has remained unclear. Here, we demonstrate through FAP-specific deletion of the Hedgehog signal transducer Smoothened that FAPs are the primary Hedgehog-responding cells during muscle regeneration. Loss of Smoothened in FAPs increases IMAT, causes persistent fibrosis, reduces the Hedgehog-dependent effectors TIMP3 and GDF10, and impairs myofiber regeneration. FAPs lacking Smoothened also fail to support in vitro myoblast differentiation and fusion as efficiently as control FAPs, showing that Hedgehog signaling helps establish a pro-myogenic FAP state early after injury. Pharmacological Hedgehog activation via the Smoothened agonist SAG fails to rescue adipocyte accumulation or myofiber regeneration when FAPs lack Smoothened. Together, these findings provide direct genetic evidence that FAPs are the primary cellular mediators of Hedgehog signaling in muscle and establish FAP Hedgehog signaling competence as a key determinant of regenerative outcome and a target for restoring muscle repair in disease.