Protein trafficking requires proper ion and pH homeostasis of the endomembrane system. The NHX-type Na⁺/H⁺ antiporters NHX5 and NHX6 localize to the Golgi, trans-Golgi network, and prevacuolar compartments and are required for growth and trafficking to the vacuole. In the nhx5 nhx6 T-DNA insertional knockouts, the precursors of the 2S albumin and 12S globulin storage proteins accumulated and were missorted to the apoplast. Immunoelectron microscopy revealed the presence of vesicle clusters containing storage protein precursors and vacuolar sorting receptors (VSRs). Isolation and identification of complexes of VSRs with unprocessed 12S globulin by 2D blue-native PAGE/SDS-PAGE indicated that the nhx5 nhx6 knockouts showed compromised receptor-cargo association. In vivo interaction studies using bimolecular fluorescence complementation between VSR2;1, aleurain, and 12S globulin suggested that nhx5 nhx6 knockouts showed a significant reduction of VSR binding to both cargoes. In vivo pH measurements indicated that the lumens of VSR compartments containing aleurain, as well as the trans-Golgi network and prevacuolar compartments, were significantly more acidic in nhx5 nhx6 knockouts. This work demonstrates the importance of NHX5 and NHX6 in maintaining endomembrane luminal pH and supports the notion that proper vacuolar trafficking and proteolytic processing of storage proteins require endomembrane pH homeostasis.

Extracellular matrix remodelling of cardiac tissue is a key contributor to age-related cardiovascular disease and dysfunction. Such remodelling is multifaceted including changes to the biochemical composition, architecture and mechanics, clouding our understanding of how and which extracellular matrix properties contribute to a dysfunctional state. Here we describe a decellularized extracellular matrix–synthetic hydrogel hybrid scaffold that independently confers two distinct matrix properties—ligand presentation and stiffness—to cultured cells in vitro, allowing for the identification of their specific roles in cardiac ageing. The hybrid scaffold maintains native matrix composition and organization of young or aged murine cardiac tissue, whereas its mechanical properties can be independently tuned to mimic young or aged tissue stiffness. Seeding these scaffolds with murine primary cardiac fibroblasts, we identify distinct age- and matrix-dependent mechanisms of cardiac fibroblast activation, matrix remodelling and senescence. Importantly, we show that the ligand presentation of a young extracellular matrix can outweigh the profibrotic stiffness cues typically present in an aged extracellular matrix in maintaining or driving cardiac fibroblast quiescence. Ultimately, these tunable scaffolds can enable the discovery of specific extracellular targets to prevent ageing dysfunction and promote rejuvenation.