Increased Proteoglycanases in Pulmonary Valves after Birth Correlate with Extracellular Matrix Maturation and Valve Sculpting

Increased mechanical forces on developing cardiac valves drive formation of the highly organized extracellular matrix (ECM) providing tissue integrity and promoting cell behavior and signaling. However, the ability to investigate the response of cardiac valve cells to increased mechanical forces is challenging and remains poorly understood. The developmental window from birth (P0) to postnatal day 7 (P7) when biomechanical forces on the pulmonary valve (PV) are altered due to the initiation of blood flow to the lungs was evaluated in this study. Grossly enlarged PV, in mice deficient in the proteoglycan protease ADAMTS5, exhibited a transient phenotypic rescue from postnatal day 0 (P0) to P7; the Adamts5^-/- aortic valves (AV) did not exhibit a phenotypic correction. We hypothesized that blood flow, initiated to the lungs at birth, alters mechanical load on the PV and promotes ECM maturation. In the Adamts5^-/- PV, there was an increase in localization of the proteoglycan proteases ADAMTS1, MMP2, and MMP9 that correlated with reduced Versican (VCAN). At birth, Decorin (DCN), a Collagen I binding, small leucine-rich proteoglycan, exhibited complementary stratified localization to VCAN in the wild type at P0 but colocalized with VCAN in Adamts5^-/- PV; concomitant with the phenotypic rescue at P7, the PVs in Adamts5^-/- mice exhibited stratification of VCAN and DCN similar to wild type. This study indicates that increased mechanical forces on the PV at birth may activate ECM proteases to organize specialized ECM layers during cardiac valve maturation.