Mending the Matrix: Piezo Initiates Transient Collagen IV Production During Basement Membrane Repair

Abstract

Basement membranes are specialized sheets of extracellular matrix that separate tissue layers and provide mechanical support to cells. Collagen IV (Col4) is the most abundant structural protein, and Col4 protomers are covalently crosslinked by Peroxidasin. Although Peroxidasin-mediated crosslinking may allow for the specialized mechanical roles of the basement membrane, the importance of this crosslinking has not been clear. We show that Peroxidasin establishes basement membrane stiffness during development and is required for both mouse and fly viability, and we also showed that continuing Peroxidasin function is essential for maintaining adult basement membranes. These findings were unexpected since Col4 is believed to be a long-lived molecule, and the covalent bond was thought to be stable and irreversible. However, our work suggests that either the covalent bond is not as stable as initially thought and needs to be replaced over time, or that Col4 is being turned over and is partially replaced requiring continual crosslinking by Peroxidasin. Basement membranes are also subject to damage, but little is known about how they are repaired, including whether and how damage is detected, what cells repair the damage, and how repair is controlled to avoid fibrosis. Using the intestinal basement membrane of adult Drosophila as a model, we found that basement membrane damage is actively detected. Following damage, there is a sharp increase in enteroblasts transiently expressing Col4, termed “matrix mender” cells. Enteroblast-derived Col4 is specifically required for matrix repair. The increase in the matrix mender cells requires the mechanosensitive ion channel Piezo, which is expressed in intestinal stem cells. Further, we found that matrix menders are induced by the loss of matrix stiffness, as specifically inhibiting Col4 crosslinking is sufficient for Piezo-dependent induction of matrix mender cells. Therefore, our data suggests that epithelial stem cells control basement membrane integrity by monitoring stiffness.