Muscle stem cells produce a protective Fibrillin-1 matrix to prevent precocious activation.

Diverse biological mechanisms regulate muscle stem cell quiescence, including inhibition of differentiation, adhesion, and translational control. Here, we identify the matrix glycoprotein Fibrillin-1 (FBN1) as a Notch-regulated, cell-autonomous effector, essential for maintaining quiescence. Known for its causal role in Marfan syndrome (MFS), our work positions FBN1 as a critical niche component that protects muscle stem cells from aberrant growth factor signaling. Targeted deletion of Fbn1 in muscle stem cells leads to dose-dependent quiescence defects, characterized by loss of cellular projections, depletion of the stem cell pool, and progressive decline in muscle function. Consistently, human MFS muscle biopsies show abnormally activated satellite cells, implicating stem cell imbalance in the development of MFS-associated myopathy. Mechanistically, the loss of FBN1 up-regulates transforming growth factor-β (TGFβ) signaling, and pharmacological blockade of TGFβ signaling rescues the cellular and physiological defects of FBN1-deficient muscle. These findings reveal a new quiescence-preserving mechanism through extracellular matrix-mediated shielding from mitogenic signals and position stem cell dysfunction as a driver of MFS myopathy.